The birthstone for the zodiac sign Libra is OPAL, a four-letter word. OPAL is a mineraloid gel which is composed of hydrated amorphous silica; its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. It is an iridescent gemstone, which means it displays rainbow-like colors when viewed from different angles. OPAL is a relatively soft stone, with a hardness of 5.5 to 6 on the Mohs scale.