Ruby

All rubies are red (pink is another matter), but not all red gems are rubies. Gem identification involves conducting tests of a gem's optical and physical properties to narrow a list of possibilities down to one. It also involves observation tempered by knowledge and experience. Oval-cut ruby, 2.16 cts, Sri Lanka. © All That Glitters. Used with permission.

Article: An Introduction to Gem Identification

Credit: All That Glitters