Sapphire is any form of corundum that is not red, as red varieties are called rubies.
Peridot is type of olivine, and comes in various shades of green, from light to a brilliant olive green.
Ruby originates from metamorphic rock, and is a variety of the mineral corundum, second only to the diamond in hardness.
When light enters the stone, it is bounced back and forth between these layers before it exits as the glowing moon-like effect we see.
Emerald is another variety of beryl. It is surprisingly common for emeralds to contain flaws and veins of chemicals called inclusions.
Diamond is a form of carbon with a tightly bound crystalline structure that originates deep inside the Earth.
Aquamarine is a variety of the mineral beryl. Beryl generally forms inside granites as magma (molten rock) cools deep inside the Earth.
Amethyst is popular for its color and crystal shapes that produce handsome, purple, sparkling clusters.
Garnet can be found in metamorphic rocks and sometimes in granites and volcanic rocks.