Opal

Opal is a unique gemstone that exhibits a phenomenon called play-of-color.  It forms when water that has seeped through ancient sedimentary rocks underground dries up,  leaving solid silica deposits in the cracks.  The silica forms sub-microscopic spheres that diffract light into its spectrum of colors, which form the play-of-color in opals.

There are many types of opal.  Those used in jewellery include: black opal, white opal, crystal opal, fire opal, and assembled opal. The value of an opal is determined by its background color, play-of-color, color range and pattern, and clarity of the stone.  Black opals are the most valuable, with the most expensive one called the harlequin.  It in particular is prized for its wide color range and brushstroke-like play-of-color pattern.  

Opals are mined in Australia, Ethiopia, and Mexico.  The New South Wales town of Lightning Ridge is the world's main source of high-quality opal.

Opal is the birthstone of October.