The Earth’s rocks are moving, squashing and often melting together. It is in this turmoil that gems are formed. The rocks are a mixture of minerals which are themselves made up of chemical elements. Atoms of chemical elements, such as carbon, oxygen and silicon, join up to make tiny building blocks. These fit together in neat regular patterns to form ‘crystals’.
Exactly what type and size of crystal forms depends on the chemical elements present, the temperature of them and the pressure exerted on them. Natural gems are found as clear or coloured crystals embedded in rock. They are also found as big crystals lining cracks or cavities in the Earth’s surface layer, or ‘crust’. Some gems are washed along by rivers and may then be picked out.
How gems are formed
Some gem crystals, like garnet, grow in solid rock as it squashes beneath moving mountains. Others, such as tourmaline, are formed in veins beneath the Earth’s surface.
Stones such as agate grow in volcanic rocks at the surface. Gems like ruby grow deep down where molten rock ‘cooks’ the Earth’s crust. Diamond crystals grow way down, around 160 km beneath the surface.