How are gems made?
What is a gemstone? A gemstone is a high concentration of a specific mineral used in jewellery when cut and polished. For example, amethyst is made out of a variety of quartz which is the most abundant mineral on the Earth’s surface, quite literally as plentiful as the grains of sand at the beach. So what makes an amethyst so much more valuable than a handful of sand? I’ve constructed an empirically produced equation to describe this:
Gemstone = mineral + “damn that do be big & shiny though”
On a serious note, while chemically unremarkable, a gemstone is a mineral produced at a uniquely large scale and quality. The basic principles by which minerals form originally from magma are fairly simple; as magma cools, minerals precipitate out in order of freezing point and either rise or sink depending on buoyancy. Imagine you’ve produced the funkiest smoothie ever whereby the fruits that compose it rematerialise within (obviously, in nature, the process is a bit more complicated). Usually, this produces a mixture of different minerals that co-exist together in solid form → rock.
When minerals condense out in large undisturbed quantities, they form gems. However, for large crystals to be produced from magma, cooling must be slow. I won’t discuss why some gems can form such intricate shapes naturally because that would require going into crystallography which is too complex for this article. Still, I may explore it in future articles.
Different gemstones form in different ways following the same principles laid out above unless described differently:
Firstly, referring back to the previous example, amethysts are produced from quartz which contains metal impurities such as iron to turn the crystal into its distinct purple. In magma, gas bubbles form and produce cavities within which quartz crystals will begin to precipitate out to form amethysts.
Rubies form in high-pressure metamorphic and igneous (form from magma as described above) environments. Metamorphism is a process whereby rocks are heated (sometimes partially melted) and/or under enough pressure to recrystallise and form a new substance/structure. Rubies are formed from the mineral corundum, where the Cr (chromium) has been substituted for Al (aluminium) to turn the gem red; all other colours of corundum gems are sapphires (sapphires don't have to be blue). An environment where rubies have formed include mountain ranges such as the Himalayas, where pressures are high, and temperatures are well over 500 degrees celsius.
Diamonds form slightly differently and are wholly carbon-based chemically; however, extreme pressure at the base of the Earths crust transform the lattice of a carbon structure(such as coal) so that each atom of carbon is chemically bonded to four other atoms of carbon to form a characteristically strong lattice (shape). Diamonds usually form in geological environments known as Kimberlite pipes. Kimberlite is an igneous rock that forms at least 100km deep; well within the Earth’s mantle.
Emeralds are a form of the mineral beryl and can form out in hydrothermal veins. Hydrothermal veins form in preexisting rock areas in which a hot mineral-rich solution has penetrated and precipitated out minerals along its path. Beryl grows in a hexagonal shape but is colourless when pure; thus, emeralds only form when chromium or vanadium is in abundance within the region where beryl forms, turning the mineral green.
Gemstones are very slightly chemically different from their normal, dull mineral parent. However, when able to form in large enough quantities can become the overpriced, fairly useless, pretty pieces of shiny rock we know and love today.
