Characteristics of Opal:

Chemical Name: Hydrous Silicon dioxide

Formula: SiO2. nH2O

Colors: Colorless, white, yellow, orange, rose-red, black, dark-blue

Structure: Amorphous

Hardness: 5-6

Specific Gravity: 1.9-2.5

Refractive Index: 1.37-1.52

Lustre: Vitreous

Streak: White

Locations: Australia, Ethiopia, USA, Mexico, Honduras, India. New Zealand

What is Opal? 

Opal falls into two categories: precious and common. The former displays highly prized rainbow iridescence with a white to dark body color, while the latter has a strong, attractive body color and no iridescence. Both kinds consist of hardened silica gel, and usually contain 5-10 percent water in submicroscopic pores. Precious Opal contains of a regularly arrangement of tiny, transparent, silica spheres, and its color play occurs when the sphere are regularly arranged and of the correct size, causing the diffraction of light and its consequent break-up into the colors of the spectrum; the actual colors that appear depend on the size of the spheres. Opal is deposited at low temperatures from silica-bearing waters, usually in sedimentary rocks. In ancient times, the primary source was in present-day Slovakia; more recently, Australia was the chief producer, and is also the source of fossil bones and seashells that have been replaced by precious Opal. Ethiopia is now the main source of gem Opal.

Fire Opal

Mineralogically, common Opal refers to fire Opals, which are transparent to translucent and do not usually show a play of colors; it can also refer to Opals with no color or transparency and no gemstone value.

Fire Opals, sometimes called jelly Opals, are prized for their rich colors: yellow, orange, orange-yellow or red. Transparent fire Opal tend to be faceted, and are often set into moderately expensive silver jewelry.