Other members of this class include coral, amber, and jet. Pearl is the birthstone for June and the Gemini zodiac sign. The crystal system is orthorhombic with a hardness slightly less than quartz. Peridot is the birthstone for August and the Leo zodiac sign. Ruby - Ruby is a term for the red gemstones derived from the mineral corundum, formed primarily from aluminum oxide.
Pure corundum is a colorless, trigonal crystal with a hardness between that of emerald and diamond.
Trace amounts of chromium are what gives the ruby its rich red color. Ruby is the birthstone for July and the Cancer zodiac sign. Several other colors of corundum are classified as sapphire such as yellow and pink. Sapphire is the birthstone for September and the Virgo zodiac sign. Topaz - Topaz is commonly found in gravel as rounded pellets, the result of being worn down by water and sand for thousands of years.