White opal, black opal, boulder opal, crystal opal, fire opal, common opal
HARDNESS & TOUGHNESS
Hardness: 5 – 6.5 Toughness: Very Poor to Fair
Australia: Black & White Opals, Brazil: White Opals, Mexico: Fire Opal, Ethiopia: White, Yellowish, Brownish
Opal is one of the most beautiful stones ever discovered, with its scintillating play of color and sheer range of hues. It’s fascinating history stretches back thousands of years and brings us right up until the present day, with significant deposits of the gem still being made around the globe. Opal is known for its unique display of flashing rainbow colors called play-of-color. There are two broad classes of opal: precious and common. Precious opal displays play-of-color, common opal does not.
Play-of-color occurs in precious opal because it’s made up of sub-microscopic spheres stacked in a grid-like pattern—like layers of Ping-Pong balls in a box. As the light waves travel between the spheres, the waves diffract, or bend. As they bend, they break up into the colors of the rainbow, called spectral colors. Play-of-color is the result.
Opal is a hydrous, silicon dioxide. It is unlike other minerals because it is not crystalline! It is considered to be a hardened jelly. It’s water content varies from about 2 to 21% mineralogically, but about 6 to 10% in gem opals.