Astro Ratnam

In modern use, the precious stones are emerald, ruby, sapphire and diamond, with all other gemstones being semi-precious.This distinction reflects the rarity of the respective stones in ancient times, as well as their quality. Gemstones are classified by their color, translucency, and hardness.

In modern times gemstones are identified by gemologists, who describe gems and their characteristics using technical terminology specific to the field of gemology. The first characteristic a gemologist uses to identify a gemstone is its chemical composition. For example, diamonds are made of carbon and rubies of aluminium oxide.