Gems are mineral crystals that are beautifully coloured or sparkling.
There are over 3000 minerals but only 130 are gemstones. Only about 50 of these are commonly used.
The rarest gems are called precious gems and include diamonds, emeralds and rubies.
Less rare gems are known as semi-precious gems.
Gems are weighed in carats. A carat is one-fifth of a gram. A 50-carat sapphire is very large and very valuable.
In the ancient world gems were weighed with carob seeds. The word ‘carat’ comes from the Arabic for seed.
Many minerals are made as magma cools. When this happens crystals, such as amethyst crystals, are formed.
Gems often form in as bubbles called geodes in cooling magma. They can also form when hot magma packed with minerals seeps up through cracks in the rock to form a vein.
When magma cools, minerals with the highest melting points crystallize first. Unusual minerals are left behind to crystallize last, forming rocks called pegmatites. These rocks are often rich in gems such as emeralds, garnets, topazes and tourmalines.
Some gems with a high melting point and simple chemical composition form directly from magma, such as diamond, which is pure carbon, and rubies.
Diamonds are among the oldest mineral crystals, over 3000 million years old.