School of rocks: Unlocking the Mineral Vault

As the Natural History Museum prepares to unlock its spectacular Mineral Vault, Helen Brown polishes up on geology and gemstones with Britain's King of Bling

Honeycomb nuggets of gold; glittering lozenges in pink, yellow or blue. Candy-cane crystals and molten-toffee slices of meteorite, shot through with chunks of green olivine. As visitors' eyes feast on the specimens in the Natural History Museum's new Mineral Vault, they may feel like they are surrounded by the world's most expensive pick'*'mix.

On the day I visit, the curator of minerals, Alan Hart, is putting the final touches to the displays. He is the man with the key to all the cases, and he lifts diamonds and rubies to the light for us to admire. The visual distraction allows me to ask a couple of embarrassingly basic questions. First: what's the difference between a rock and a mineral?

"A mineral is a naturally occurring chemical compound," says Hart. "There are about 4,500 known in the world – we call them 'species'. A rock is made of lots of minerals together. Although, confusingly, some are just composed of just one [monomineralic], most are an amalgamation of about 10."

The super-secure Mineral Vault showcases the best specimens in the museum's collection. It places cut and set gemstones alongside their naturally occurring forms. "Mineralogy is a science with its roots in chemistry, but it's so deeply entangled with cultural history and art," says Hart. "Our subjects are so beautiful, interesting and often sculptural. As their values and uses have changed over time, they tell us as much about the nature of our own species as they do about themselves. We preserve many of these samples as objects – we wouldn't work or experiment on them."

Carefully lifting the diamond-starred Murchison snuffbox from its case, Hart talks about the history of diamonds. They are carbon allotropes that take their names from the greek adamas, meaning invincible, and tell us a great deal about the conditions of our planet's formation.

"Seventy-five per cent of the diamonds in the world are 3 billion years old," says Hart. They're formed from carbon under extreme pressure of between 45 and 60 kilobars, but at a comparatively low temperature range of between 900deg and 1,300deg. Such conditions exist in only two places on Earth: in the lithospheric mantle below continental plates, and at the site of meteorite strikes.

Other treasures in the museum's mineralogic chest hail from those same meteors. Dr Caroline Smith, the curator of meteorites, tells me that between 40,000 and 60,000 tonnes of space rock hits the earth each year, although "most of that mass is made up of tiny dust specks. And most fall into the sea. We do get around 1,000 basketball-sized meteorites a year, too."

Many of the stones that Hart raises lovingly to the light have names that would befit a footballer's wife: Sherry Topaz, Pink Beryl and Watermelon Tourmaline. "The complex chemistry of tourmaline meant that this one started off as a nice red crystal (rubelite)," he explains, "and then the chemistry of the contributing fluids change – there might have been slightly more iron – and that absorbs the light differently to give us the green."

He reaches for a gem that resembles a fruity cocktail fused into a glittering ice cube. "This is one of our treasures," he says. "Padparadscha – an orange sapphire with a hint of pink. Stones of this quality are very rare. Usually you'd see them with one or two carats. This one is 57. Some of the rarest things we have are rubies as they come out of the ground," he gestures towards a slab of rich, murky red. "Burma is the place with the finest rubies – called 'pigeon's blood' rubies."

Downstairs, Hart clicks open a safe to hand me a chunk of gold called the La Trobe Nugget. As I feel the soft metal, he explains that it is "one of the finest crystalline golds in the world – look at the cubes in it. They're rounded, but grow from octahedrons – a cubic structure. And come look at the alexandrite..."

Hart is a bit like Gollum, with his "precious", but this is one of the best specimens of alexandrite ever found. As he holds it up to the light pouring in through the window, it appears green. Put it under artificial light, however, however, and it does a little magic trick – it glows a beetroot red. "We were experimenting with this bit this morning," he says. Scientists are still studying why light frequencies cause this effect in the stone.

Minerals could hold the key to all sorts of future technological advances. We all know that diamonds – once valued only for their beauty – turned out to be the ultimate mining tools, and have properties that are now seeing them used in groundbreaking medical techniques and in electronics. " There are always new uses for minerals," says Hart. Some minerals, such as asbestos, seem to promise much, then turn out to have hidden dangers. "But who would have thought, 100 years ago, that a mineral like silicon would be everywhere today, so essential to our daily lives?"

Set in stone: highlights from the Mineral Vault

This world-class collection of 296 naturally coloured diamonds has been loaned to the Natural History Museum by collectors Alan Bronstein and Harry Rodman. It comprises 267.45 carats of exceptionally rare stones from the 12 colour varieties, from emerald green to blood red. The display highlights a property shared by coloured and white diamonds – some glow and change colour when exposed to ultraviolet light.

1. La trobe Gold Nugget

Charles Joseph La Trobe, lieutenant-governor of the state of Victoria, Australia, was visiting the McIvor gold mine when word came that an unusually large nugget had been found and named in his honour. although small as nuggets go, it is one of the largest crystalline-fold specimens in existence. Some of the cubes are more than half an inch wide, and the whole is comprised of a mass of crystals. The richness of the colour is due to a small impurity of copper. Gold is one of the few metals found in nature that is generally untainted with other elements. Crystal forms are extremely rare – and this cluster of intergrown cubic crystals is one of the world's finest examples.

2. Alexandrite

Discovered in 1830 in Russia, alexandrite is a stone that changes colour – generally, from green to red. Since these were the Russian imperial colours, it was named after the Tsar Alexander II. Alexandrite emerged millions of years ago in a metamorphic environment. Its formation required specific geological conditions: the elements beryllium and chromium – the colouring agent in alexandrite – have contrasting chemical characteristics and do not, as a rule, occur together. A deficiency of silica, the second most common element in the Earth's crust, is also required. This geological scenario has occurred only rarely in the Earth's history and, as a result, alexandrite crystals are extremely rare.

3. Morganite

Morganite is the term for pink beryl (yellow being "heliodor", green "emerald" and blue "aquamarine". Beryl forms in prismatic crystals that can vary hugely in size: one weighing 200 tons and measuring 20ft was mined in Brazil. Beryl is the main commercial ore of beryllium – a metal used to make corrosion resistant springs, electrical contacts and cladding for nuclear fuel elements.

4. Boulder Opal

Opals range in colour from fiery red to deep black. What they all have in common is a bewitching flash of rainbow colours, or iridescence. This inspired an Aboriginal legend that their creator came to earth on a rainbow, to bring peace to humanity. Opals are 5-10 per cent water and some may crack if left to dry out too quickly. The iridescence is caused by the reflection and scattering of light from minute, uniform and closely packed silica spheres. Since the late 19th century, Australia has been the principal source of black and white opals.

5. Topaz

It is widely believed to have derived its English name from the Sanskrit word "tapas" meaning "fire". Topaz forms in igneous rocks such as granite and pegmatite and may also be found in waterworn pebbles in alluvial sands. Some of the most sought after topaz gemstones contain tear shaped internal cavities containing immiscible gas bubbles or liquid.

6. Siderite Cube

Known affectionately as "The Box" at the Natural History Museum, this sculptural cube of the iron carbonate siderite was found in Devon's Virtuous Lady Mine (named after Elizabeth I) in the early 1830s. " What's so great about this specimen," says Hart, "is that it's an example of chemistry at work in the Earth and it's a geological mystery. The siderite would have formed around fluorite crystals. Those are very hard. Siderite is relatively soft. So the question is: what circumstances caused the fluorite to dissolve and leave the lovely, fairy-box siderite? It's the reason we opened the Mineral Vault. Maybe some eight-year-old girl or boy will see it and be inspired to solve the riddle."

7. Padparadscha

This exotic name is given to a rare type of sapphire that has a delicate, orange-pink hue. The colour of a true padparadscha is disputed: if it is too orange, it is merely an orange sapphire. The perfect colour is that of a Sri Lankan sunset. Hard-wearing and heat-resistant, sapphires of every shade have been mined for use in the mechanisms of clocks and measuring instruments. They are also used in window apertures and the nibs of ball-point pens.

The exhibition opens on 28 November

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