Opal is a hydrated amorphous form of silica; its water content may range from 3 to 21% by weight, but is usually between 6 and 10%.
Because of its amorphous character, it is classed as a mineraloid, unlike the other crystalline forms of silica, which are classed as minerals
Opal can occur in many colors including white, colorless, pale yellow, pale red, gray, brown and black. Diffraction can cause flashes of any color of the rainbow which can be seen in opal's color play. Yellow, blue and green are most common, whereas violet, red and orange are the rarest colors seen through play of color.
Opalescence is usually milky blue in color, but it can occur in rarer colors. The saturation of body color, the play of color and the transparency determine an opal's value.
Pictured: Stacking Ring Set created in our Beginner Metalworking 101 DVD Series with an opal stone set in the center ring.