Only five types of gemstones were considered precious: diamond, ruby, sapphire, emerald, and amethyst.

What specifically are gems and gemstones? We know some of the most common ones - emeralds, rubies, sapphires and diamonds. But what exactly are they, how are they made and why we do we prize them so highly?

The International Gem Society defines a gem as "a mineral that has been chosen for its beauty and durability, then cut and polished for human adornment."

Most gems are minerals. Others, like pearls or amber, are created from living organisms. Usually gems are of a color that mirrors other natural elements we consider beautiful or valuable. Emeralds mirror the lush green of plant life, sapphires are the color of the sky and rubies are a rich blood-red. There are variations in color in all of these gems, of course, and in diamonds, but those are the colors most normally associated with them. Diamonds can come in colors as well, with pink and canary diamonds being highly prized variants.

Most gems are hard and durable, although pearls can be easily damaged, as can opals and moonstones. Usually a gemstone has to be durable enough to wear as adornment, but others, while too soft to wear, are valued by collectors. Pearls can eventually disintegrate, even if all you do is wipe dust off them. And if you spray hairspray or perfume on them, this can also damage your pearls.

Opals are highly prized for their beauty and fire and make exquisite jewelry, but are extraordinarily susceptible to damage. They can shatter easily if dropped. And there's a story of a woman who wore an opal brooch to a holiday party. When she left the warmth of the party to leave and the brooch experienced the sudden drop in temperature, it shattered, just like a glass that's been heated and then cooled too quickly.