Amgueddfa Blog: Geology

2019 is the 150th anniversary of the Periodic Table of Chemical Elements (see UNESCO The "International Year of the Periodic Table of Chemical Elements (IYPT2019)" is an opportunity to reflect upon many aspects of the periodic table, including the social and economic impacts of chemical elements.

Sulphur is the fifth most common element (by mass) on Earth and one of the most widely used chemical substances. But sulphur is common beyond Earth: the innermost of the four Galilean moons of the planet Jupiter, Io, has more than 400 active volcanoes which deposit lava so rich in sulphur that its surface is actually yellow.


The sulphate salts of iron, copper and aluminium were referred to as “vitriols”, which occurred in lists of minerals compiled by the Sumerians 4,000 years ago. Sulfuric acid was known as “oil of vitriol”, a term coined by the 8th-century Arabian alchemist Jabir ibn Hayyan. Burning sulphur used to be referred to as “brimstone”, giving rise to the biblical notion that hell apparently smelled of sulphur.


Sulphur rarely occurs in its pure form but usually as sulphide and sulphate minerals. Elemental sulphur can be found near hot springs, hydrothermal vents and in volcanic regions where it may be mined, but the major industrial source of sulphur is the iron sulphide mineral pyrite. Other important sulphur minerals include cinnabar (mercury sulphide), galena (lead sulphide), sphalerite (zinc sulphide), stibnite (antimony sulphide), gypsum (calcium sulphate), alunite (potassium aluminium sulphate), and barite (barium sulphate). Accordingly, the Mindat (a wonderful database for all things mineral) entry for sulphur is rather extensive:


Sulphur is the basic constituent of sulfuric acid, referred as universal chemical, ‘King of Chemicals’ due to the numerous applications as a raw material or processing agent. Sulfuric acid is the most commonly used chemical in the world and used in almost all industries; its multiple industrial uses include the refining of crude oil and as an electrolyte in lead acid batteries. World production of sulfuric acid stands at more than 230 million tonnes per year.


Gunpowder, a mixture of sulphur, charcoal and potassium nitrate invented in 9th century China, is the earliest known explosive. Chinese military engineers realised the obvious potential of gunpowder and by 904 CE were hurling lumps of burning gunpowder with catapults during a siege. In chemical warfare, 2,400 years ago, the Spartans used sulphur fumes against enemy soldiers. Sulphur is an important component of mustard gas, used since WWI as an incapacitating agent.


Sulphur-based compounds have a huge range of therapeutic applications, such as antimicrobial, anti-inflammatory, antiviral, antidiabetic, antimalarial, anticancer and other medicinal agents. Many drugs contain sulphur; early examples include antibacterial sulphonamides, known as “sulfa drugs”. Sulphur is a part of many antibiotics, including the penicillins, cephalosporins and monolactams.


Sulphur is an essential element for life. Some amino acids (cysteine and methionine; amino acids are the structural components of proteins) and vitamins (biotin and thiamine) are organosulfur compounds. Disulphides (sulphur–sulphur bonds) confer mechanical strength and insolubility of the protein keratin (found in skin, hair, and feathers). Many sulphur compounds have a strong smell: the scent of grapefruit and garlic are due to organosulfur compounds. The gas hydrogen sulphide gives the characteristic odour to rotting eggs.


Sulphur is one of the essential nutrients for crop growth. Sulphur is important to help with nutrient uptake, chlorophyll production and seed development. Hence, one of the greatest commercial uses of sulfuric acid is for fertilizers. About 60% of pyrite mined for sulphur is used for fertilizer manufacture – you could say that the mineral pyrite literally feeds the world.


Use of sulphur is not without problems: burning sulphur-containing coal and oil generates sulphur dioxide, which reacts with water in the atmosphere to form sulfuric acid, one of the main causes of acid rain, which acidifies lakes and soil, and causes weathering to buildings and structures. Acid mine drainage, a consequence of pyrite oxidation during mining operations, is a real and large environmental problem, killing much life in many rivers across the world. Recently, the use of a calcareous mudstone rock containing a high proportion of pyrite as backfill for housing estates in the area around Dublin caused damage to many houses when the pyrite oxidised; the case was eventually resolved with the “Pyrite Resolution Act 2013” allocating compensation to house owners.

Conservation of museum specimens

Because iron sulphides are highly reactive minerals, their conservation in museum collections poses significant challenges. Because we care for our collections, which involves constantly improving conservation practice, we are always researching novel ways of protecting vulnerable minerals. Our current project, jointly with University of Oxford, is undertaken by our doctoral research student Kathryn Royce

Come and see us!

If all this has wetted your appetite for chemistry and minerals, come and see the sulphur and pyrite specimens we display at National Museum Cardiff, or learn about mining and related industries at Big Pit National Coal Museum and National Slate Museum

On the 22nd June our new summer exhibition opened. This family friendly exhibition runs until September and delves into the captivating life of snakes, helping you to find out more about these extraordinary and misunderstood creatures. We are hoping to feature more detailed stories about all of the things mentioned below in a series of blogs running through July and August so keep tuning in to find out more.

Dr Rhys Jones at our opening launch event.

Snakes is a touring exhibition created by a company called Blue Tokay with added bonus content generated by our team. Work began on bringing together all of this way back in September 2018 and since then we have been busy researching, writing text and preparing some great specimens for you all to enjoy.

The main exhibition covers all aspects of the lives of snakes, so we focused our efforts on highlighting our collections at the museum. We hold over 3.5 million natural history specimens here, and as you can imagine, not everything is on display. We hold a small collection of 500 reptiles from all over the world. These are mostly preserved in alcohol and stored in jars, but we also have skeletons, skins and eggs. We chose 32 of our best snakes to go out on display. Each of these were carefully rehoused and conserved as many of the specimens were old and in need of work.

Some of the fantastic snake collections at the museum.

Our Conservation intern, Caitlin Jenkins, hard at work rehousing the snakes.

But it’s not just snakes in jars. We have also displayed some fantastic casts of 49 million years old fossil snakes, and 3D printed the vertebra of Titanoboa, the largest snake that ever lived.

Snake evolution case featuring casts of snake fossils.

One of my favourite features of the exhibit are our objects dealing with snake folklore and mythology, featuring a 13th century manuscript showing how snakes were used in medicinal remedies. Also some fantastic ‘snakestones’, actually fossil ammonites with snake heads carved on to the top.

Getting out the Snakestones from the collections.

You may also recognise the statue of Perseus that has long been displayed in our main hall. Perseus is enjoying his new surroundings, with Medusa’s snake ridden head looking positively sinister with the new lighting.

Perseus with the severed head of the serpent haired Medusa.

The exhibition features six live snakes and as I’m sure you can imagine, bringing live animals into a museum requires a LOT of preparation. We have done a great deal of work to ensure that their time with us is spent in 5 star accommodation. Their ‘vivaria’ are purpose built to ensure our snakes are well cared for, including warm and cool spots, as well as a water feature for a bathe. We have a fantastic (and very brave) set of staff who are volunteering their time to looking after them including changing water bowls, and clearing up their poo! Dr Rhys Jones (Cardiff University) has been fantastic with helping throughout this whole process, including coming in every week to feed them. The snakes are all provided by a company called Bugs n Stuff, you can see a video of them installing the live snakes here.

The largest of our live snakes, Prestwick, the Jungle Carpet Python.

Dr Rhys Jones with some of our staff at the live snake care team meeting.

Guy Tansley from Bugs N Stuff with Mela, the Boa constrictor.

Finally, our fantastic learning department, design team and technicians have worked hard to add some fun activities for all to enjoy. Our Spot the Snake pit features, amongst other things, two beautifully conserved models of a cobra and a rattlesnake that date back to 1903, and a real freeze-dried adder! We also have a snake expert quiz, a world map of snakes, and drawing and colouring stations. Volunteers will be in the gallery periodically across the summer with snake handling specimens including a real full length skin of an African Rock Python.

The exhibition runs till 15th September 2019, entry charges do apply, and all your contributions go towards bringing you even bigger and better exhibitions in the future. Please note that there is no live handling of the snakes within the exhibition, there will be a series of bookable handling sessions throughout the summer as well as a Venom themed Open Day in August. To find out more about all of this, go to our What's On page.


Into June and we have selected silicon as our element of the month. This element might not be instantly recognisable as of significance to Wales, but it does have an interesting history.

Silicon (chemical symbol – Si), atomic number 14, is a hard but brittle crystalline solid, with a blue-grey metallic lustre. Silicon is the second most abundant element (about 28% by mass) in the Earth’s crust after oxygen with which it has a strong affinity. Consequently, it took until 1823 for a scientist - Jöns Jakob Berzelius – to prepare it in pure form.

In Wales, silicon is present virtually everywhere in one form or another: from quartz (silicon dioxide, SiO2) in sedimentary siltstones, sandstones and conglomerates; complex silicates in igneous and metamorphic rocks; to sediments in soils.

Silica (silicon dioxide, or quartz) was mined extensively in the Pontneddfechan area, in South Wales, from the late 18th century until 1964 for the manufacturing of firebricks for kilns and furnaces. It occurs as a very pure material highly concentrated in quartzite within a geological unit known as the Basal Grit. Weathering and erosion of the quartzite has produced deposits of silica sand and this was extensively quarried for the production of refractory fire bricks for the smelting industries.

In North Wales, a little-known trade in rock crystal – a colourless, glassy variety of quartz crystal – took place in Snowdonia during the 18th and 19th centuries centred upon the village of Beddgelert. T. H. Parry-Williams refers to this in one of his writings. Miners and mountain guides searched for veins of quartz in the mountains and collected crystals to sell to tourists as curios and some were possibly used to make crystal chandeliers. Later, crystals were occasionally discovered in the vast slate quarries, or during the large-scale construction of forestry tracks during the 1960s.

Silicon, as silica (another name for silicon dioxide) is also important to certain organisms. In particular diatoms and sponges.

Diatoms are single-celled microscopic algae with a complex cell wall made of silica. They are abundant in all waters, produce oxygen and are food for other aquatic organisms. Diatoms are also frequently used to monitor water quality.

Sponges build their skeletons from a framework of tiny elements called spicules, which are made of silica in most sponge groups.  One of the most beautiful examples is the Venus’ Flower Basket glass sponge, which lives anchored to the deep ocean floor near the Philippines.  A pair of shrimps lives inside this sponge, breeding inside it and spending their whole lives protected within its delicate glass walls.  Thanks to this unusual symbiotic relationship, the dead skeletons of Venus’ Flower Baskets are a popular wedding gift in Japan.

Sponges are the most primitive kind of animal on Earth, and their resistant spicules are found as fossils from as far back as 580 million years ago. Silica is also important in the preservation of other types of fossil.  When dead animals or plants are buried, silica from groundwater can fill in the pores and other empty spaces in wood, bone or shells, and/or it can replace the original remains as they decay or dissolve.  This is most common in areas where the groundwater has high silica levels, due to volcanic activity or erosion of silica-rich rocks.  The organic remains act as a focal point for silica formation, and often the rock surrounding the fossils is made of different minerals.  For example, shells that were originally made of calcium carbonate can dissolve and be replaced by silica, whilst being fossilised within limestone (calcium carbonate).  Extracting the fossils is a simple process of putting the rock in some acid and waiting for it to dissolve, leaving behind the silicified fossils.  The Museum’s fossil collections include many silicified shells of brachiopods, ammonites, bryozoans and other sea creatures.

One of the most spectacular types of fossil preserved in silica is ‘petrified wood’.  Silica replaced the original cells of the wood as it decayed and also filled in any gaps, literally ‘turning it to stone’.  In some places, including Patagonia and the USA, whole tree trunks replaced by silica are found in so called ‘petrified forests’.  Other plant fossils, such as cones, can also be fossilised in this way.

Chert is a rock made of very small crystals of silica.  Many major chert deposits formed at the bottom of ancient oceans from ‘siliceous ooze’, which is made of the skeletal remains of millions of tiny organisms including diatoms and radiolarians (single-celled plankton).  Chert nodules can also form within other rocks through chemical processes. 

Chert found within chalk is known as flint, and was a very important material for making tools throughout Prehistory. Tools are made by knapping, that is striking a prepared flint edge, or striking platform, with a harder stone to detach pieces called flakes or blades. These flakes, blades, and indeed the core from which they are struck can then be modified with secondary working into fine tool forms. Amongst the most skilful are fine arrowheads, including these from a Bronze Age grave at Breach Farm, Vale of Glamorgan, Wales. Flint was generally the material of choice for making sharp cutting tools as it is so fine-grained and fractures conchoidally and cleanly it gives a really sharp cutting edge. Indeed, so much so, that anecdotally eye-surgeons are reported to occasionally use a freshly struck flint blade in the operating theatre!

Because it is very fine-grained and hard, chert can preserve fossils of very small things from far back in our planet’s history.  The oldest potential fossils on Earth are found in cherts, and include the possible remains of bacteria from over 3 billion years ago.  Younger fossils, from the Rhynie Chert of northern Scotland, provide a glimpse of one of the earliest land communities, 400 million years ago.  Simple plants, and animals including primitive spider-like creatures and scorpions, were preserved in fine detail thanks to silica-rich water from volcanic hot springs.

Opal is a hydrated form of silica, meaning that it contains between 3 and 21% water.  Unlike standard silica, it does not have a set crystal form, but some of its forms diffract light, creating a beautiful iridescent effect in a variety of different colours.  For this reason, opal has been prized for centuries as a gemstone for making pendants, rings and other jewellery.  Australia produces a lot of the world’s opal, and is also a source of rare and spectacular opalised fossils.  The shells of invertebrates such as belemnites (prehistoric squid-like creatures), and even dinosaur bones, have been replaced by opal, creating very colourful specimens in a world where fossils are usually grey or brown.

Continuing the international year of the periodic table of chemical elements, for May we have selected lead. Everyone knows that lead is heavy, or more correctly dense, but did you know how important it was to the Roman Empire?

Mad, bad and dangerous to use – lead in Roman times

In Roman times lead was used extensively throughout the Empire. The chemical symbol for lead is Pb which comes from the Latin word for lead, plumbum, and is also the derivation of the word ‘plumbing’.

The extraction of lead ore is reasonably straightforward, and there was an abundant supply within the ever expanding empire. As well as being easy to find and one of the easiest metals to extract, lead is soft and malleable, has a relatively low melting point of 327.5°c (“low enough to melt in a camp fire”) and it is much denser and heavier than other common metals. It is also possible to cast it. This meant that it was widely produced and used for an enormous range of purposes from industrial to domestic.

The Romans were famous for their plumbing systems, and as lead pipes replaced older constructions made of stone and wood, ever more elaborate plumbing systems could be created. In 2011 during the excavation undertaken by Cardiff University of the Southern Canabae at Caerleon an example of a lead water pipe was excavated close to the amphitheatre. It has a diameter of 0.12m, is bulged in the middle where two lengths of pipe have been joined using a wiped joint, and there are remains of a round collar pierced by iron nails at one end (to the left in the image) where it was probably attached to a wooden tank or pipe. As can be seen, the main pipe has been tapped by a narrower pipe and they were presumably used to feed a fountain or water feature within the large courtyard building found alongside.

The malleable nature of lead, and its relatively low melting point also made it very useful for soldering and repair work, for architectural fittings and for lining containers. It was even used as a type of Rawlplug.  Its density made it ideal for weights and its abundance made it cheap enough to be used in a whole range of everyday objects, from a variety of containers, to lamps used to light Roman homes, luggage labels and stamps of all varieties. It was used in paint and in medicines and cosmetics and it was even used to sweeten and colour wine. Perhaps most important of all most ores of lead also contain a small quantity of silver and, in some instances the value of silver outweighed the value of lead. For an economy so dependent on silver this precious by-product was of great importance.

A perfect example of the everyday use of lead is the Roman lead bread stamp from Prysg Field in Caerleon (see the image on the right). Within the Fortress bread was baked in a communal oven and the bread stamps were used by each Company to clearly identify their bread ration for the day. The one in the photograph below was for the ‘Century of Quintinus’.

The simple lead lamp from Gelligaer near Caerphilly (shown on the right) illustrates a common form found on Roman sites – cheap and utilitarian. The main tray would have been filled with tallow (animal fat) and a wick would have extended to the raised area.

The wonderful curse tablet (shown on the right) found at Caerleon is the only such object so far recovered from Wales. It clearly illustrates the malleable and soft nature of the metal but also links to its cultural status as a ‘base’ metal. Scratched into the surface of the lead is a curse invoking the aid of the goddess Nemesis against a thief of a cloak and boots.

Research has shown that the letters of the inscriptions at Caerleon were painted exclusively using a red pigment called litharge (PbO), also known as red lead. Traces of red can still be seen on a stone inscription found at the Amphitheatre in Caerleon (shown on the right).

The heavy quality of lead meant that the Romans used it to make weights or to weigh things down. A Mediterranean style anchor stock from a small cargo ship was found just off the coast of the Llyn Peninsula at Porth Felen, Aberdaron.

The Romans cast the lead they mined into ingots known as ‘pigs’. Originally the lead mines in Britain were under the direct control of the Roman Authorities but this responsibility was later handed over to trusted local agents who leased the lead mines out to local companies on payment of a levy. A Roman lead pig found at Carmel in Whitford shows the insignia of one such agent, Gaius Nipius Ascanius. Other examples found in Britain have marks such as “EX ARG” (Ex argentariis), indicating that it was from a lead-silver works, or Deceangl[icum] indicating it was lead from the Deceanglic (Flintshire) region.

Lead in Roman Wales

According to Pliny, lead “is extracted with great labour in Spain and throughout the Gallic provinces; but in Britannia it is found in the upper stratum of the earth, in such abundance, that a law has been spontaneously made, prohibiting any one from working more than a certain quantity of it.” (Natural History, Book 34, Chapter 49)

Lead was so important to the Romans that they started extracting the ore almost as soon as they arrived in Britain. The Mendip area around Charterhouse in modern day Somerset was an important area for lead mining and evidence shows that mining began here as early as AD49. Originally mining was under the control of the Army, and in the Mendip Hills at this time that was the Second Augustan Legion. Their experience in overseeing the extraction of lead mines may well have proved useful when the Legion transferred to their new headquarters in Caerleon in AD74/5.

The area around Draethen woods near Lower Machen, contained lead ore reasonably high in silver content, certainly comparable to the Mendips and higher than any other lead ores to be found in South Wales. Draethen is just 10 miles away from Caerleon, about the same distance from the Roman fort at Gelligaer and even closer to the fort at Caerphilly. In 1937 work on the construction of a new bypass in Lower Machen uncovered evidence of Roman occupation, including evidence of a working floor with layers of charcoal and numerous pieces of lead and lumps of lead ore.  Nash-Williams (Archaeologia Cambrensis, 1939) states that the early date of the pottery and coin finds suggest that Lower Machen “was certainly in Roman hands by the time of the completion of the Roman military conquest of South Wales in AD75, possibly before”. More recent discoveries of pottery also suggest an occupation period of c AD 70-100.

The 1965 exploration of ‘Roman Mine’ in Draethen throws more light on Roman lead mining in this area. In Roman times lead ore was extracted by laying wooden fires against the rock, heating the rock to a high temperature and then throwing cold water or vinegar onto it. This caused the rock to split into smaller pieces which could then be sorted by hand within the mine. The waste, known as ‘deads’, was packed into side chambers and empty crevices and the ore was brought to the surface on wooden trays or in leather sacks and wooden buckets. The evidence found within Roman Mine exactly matches this technique. Charcoal was found throughout the mine, even in the smaller tunnels, and the side chambers were filled with waste. The walls and roof of the tunnels were covered in a thick black patina caused by the production of an enormous amount of smoke. The creation of so much smoke also meant that the Romans had to sink shafts at regular intervals in order to create a through draft and the main passage of Roman Mine has many such outlets. No tools were found in Roman Mine but pick marks are frequent throughout the tunnels.

Who worked these mines? Nash-Williams (Archaeologia Cambrensis, 1939) presumes that the labourers who worked in Draethen were “slaves and convicts working under the supervision of a military guard, and the settlement would be under the control of an officer or government official.” It is likely that the lives of these miners were short and judging by the very small spaces of some of the worked areas within Roman mine it is possible that some of the labourers were children.

A detailed report on the lead mines in Draethen can be found here and more information about the objects found during the exploration of the mine can be found here.

Control of Substances Hazardous to Health (COSHH) - Lead poisoning in Roman times

Lead, despite all its many useful qualities, is also toxic. When ingested or inhaled, lead enters the bloodstream and inhibits the production of haemoglobin which is needed by red cells to carry oxygen. When lead levels in the blood increase it has a devastating effects on the body, including irreversible neurological damage. Children are particularly vulnerable because their tissue is softer and their brains are still developing.

Contemporary writing makes it clear that Romans were aware of the dangerous effects of lead and knew it could lead to insanity and death.

Pliny, in his Natural Histories, wrote about the noxious fumes that emanated from lead furnaces. Vitruvius in De Architectura suggests the use of earthen pipes to convey water because “that conveyed in lead must be injurious to the human system.” He goes on to note that “This may be verified by observing the workers in lead, who are of a pallid colour; for in casting lead, the fumes from it fixing on the different members, and daily burning them, destroy the vigour of the blood; water should therefore on no account be conducted in leaden pipes if we are desirous that it should be wholesome.” Celsus in De Medicina urges the use of rain water, conveyed through earthen pipes into a covered cistern.

However, although some advised against its use it was such a commonly used and important metal that it was almost impossible to manage without it. The vast majority of Romans probably remained unaware of the dangers and continued to use it in their everyday lives.

The study of lead levels in individuals from the Romano-British period is enabling researchers to gain a greater understanding of normal levels for different regions, and allows a growing degree of confidence in the identification of immigrants into an area.  In complementary isotope studies the remains of the man found in the coffin at Caerleon showed lead concentrations of 4 parts per million (ppm), preserved in his teeth, which is typical of someone from the local area at this period.

Lead pollution in Ancient times.

The Romans’ extensive use of lead gives us a fascinating insight into the ups and downs of Roman history. In 2018 analysis of cores taken from Greenland’s ice sheet by the Norwegian Institute of Air Research showed that environmental pollution is not just a modern phenomenon. Pollution from lead mines can be traced in the layers of ice and clearly show the pollution left behind in ancient times. The researchers were able to use their measurements of lead pollution to track major historical events and trends. A clear pattern emerges where lead pollution drops during times of war, as fighting disrupts lead production. It then increases during periods of stability and prosperity.  Lead pollution rose dramatically from the end of the Roman Republic and through the first 200 years of the Roman Empire, the Pax Romana. The measurements also starkly show the fall of this great Empire. The Antonine Plague struck in AD 165, a devastating pandemic that historians think was either smallpox or measles. Almost five million people died over the 15 years that the plague raged in the Empire, and whilst the Empire continued after the plague came to an end its economy never recovered. This is clearly shown in the low levels of lead in the ice layers during the years of the Plague and the centuries following it. The high lead emissions of the Pax Romana end at exactly the same time as the plague struck and do not reach those same levels again for more than 500 years.

More information about this fascinating research can be found here.

The museum’s geology collections contain many examples of lead ores from Wales and around the world including lead sulphide, or galena, the main ore of lead. Post-1845 (when official records started being kept) in excess of 1.2 Million tons of lead concentrate was produced from Welsh mines, but with a history of mining dating back to at least Roman times that figure should be considerably greater.

Natural weathering and oxidation of lead ores results in the formation of some beautifully coloured minerals. A few examples are illustrated here. It should also be noted that not all lead-bearing minerals are toxic – certain compounds containing lead are very stable. Experiments have shown that polluted mine dumps containing lead can be stabilised by oxidising some of the lead into phosphates such as pyromorphite or plumbogummite.

Continuing the international year of the periodic table of chemical elements, for April we have selected Calcium. Known by most as the fundamental element in bone-forming or limestone, it has a host of other applications and is present in seabeds and marine life past and present.

Calcium (Ca) is a light-coloured metallic element with an atomic number of 20.  It is crucial for life today and commonly forms a supporting role in plants and animals. The 5th most common element in the earth’s crust, calcium forms many useful rocks and minerals such as limestone, aragonite, gypsum, dolomite, marble and chalk.

Aragonite and Calcite, the two most commonly crystalised forms of calcium carbonate, helped form the 2 million shells in our mollusc collection, the core of which is the Melvill-Tomlin collection, donated to the museum in the 1950s. An international collection it contains many rare, beautiful and scientifically important specimens and is utilised by worldwide scientists for their research. Pearls, also made of aragonite and calcite, are produced by bivalves such as oysters, freshwater mussels and even giant clams. In nature pearls are the result of the molluscs’ reaction against a parasitic intruder or a piece of grit. The mantle around the soft bodied animal secretes calcium carbonate and conchiolin that surrounds the invading body and imitates its shape so they are not all perfectly spherical. In the pearl industry the oyster or mussel is ‘seeded’ with a tiny orbs of shell to ensure that the resulted pearl is totally spherical.

Mollusc shells are created as protective shields by their soft-bodied owners and this is true of other invertebrates, especially in the world’s oceans. Coral reefs and some marine bristle worm tubes (Serpulidae, Spirorbinae) rely on the reinforcing nature of calcium carbonate to provide support and protection to their soft bodies. Crustaceans such as crabs and lobsters have a hard exoskeleton strengthened with both calcium carbonate and calcium phosphate. Calcium required after moulting in lobsters, crawfish, crayfish and some land crabs is provided by gastroliths (sometimes referred to as gizzard stones, stomach stones or crab’s eyes). They are found on either side of the stomach and provide calcium for essential parts of the cuticle such as mouthparts and legs. The museum’s collections holds nearly 750,000 marine invertebrates, including crustaceans, corals and bristleworms.

Many of the 700,000 fossils in the Museum’s collections are also made of calcium minerals.  Invertebrates use two main forms of calcium carbonate to make their shells and exoskeletons, and the one they use influences how likely they are to be immortalised as fossils.  Aragonite, found in the shells of molluscs such as ammonites, gastropods and bivalves, is unstable and doesn’t usually survive for millions of years.  During fossilisation, aragonite shells either dissolve away completely, or the aragonite recrystallizes to form calcite.  Calcite was used to make the shells and skeletons of extinct groups of corals, articulate brachiopods, bryozoans, echinoderms and most trilobites.  It is much more stable than aragonite, so the original hard parts of these creatures are commonly found as fossils, millions of years after they sank to the sea floor.  Large calcite crystals are often found filling spaces in fossils, such as the chambers inside ammonite shells.  Vertebrates use a different calcium mineral to make their bones and teeth: apatite (calcium phosphate), which can survive for millions of years to make iconic fossils such as dinosaur skeletons and mammoth tusks.

The Museum’s rock collections contain many limestones, rocks formed at the bottom of ancient seas from bits of shells and other calcium carbonate-rich remains.  For millenia, people have used limestones as a construction material: from carved stone in the iconic Greek and Roman temples; broken fragments as ballast in the base layer of railways and roads; or burnt to form lime in the manufacturing of cement.  National Museum Cardiff and other iconic buildings in Cardiff Civic Centre were built from a famous Dorset limestone called Portland Stone.  The Museum’s floor is tiled with marble, limestone that has been transformed (‘metamorphosed’) under great heat and pressure.  Marble has long been prized by sculptors, since the ancient Greeks and Romans. The Museum’s art collections include works in this material by Auguste Rodin, John Gibson, Sir Francis Chantrey, Sir William Goscombe John, and many others. There are also important examples of work by twentieth-century sculptors, such as Jacob Epstein, Eric Gill and Henri Gaudier-Breszka. They preferred carving the softer texture and density of the softer limestone, Portland Stone and sandstone.