Birthstone: Opal
Opal is a hydrated, amorphous form of silica. Its water content can range from 3 to 21% by weight but is usually between 6 and 10%.
Because it lacks a crystalline structure, Opal is classified as a mineraloid rather than a true mineral like other forms of silica.
Opal occurs in a wide range of colors including white, colorless, pale yellow, pale red, gray, brown, and black. Diffraction within the stone can create flashes of rainbow colors-this is known as its "play of color."
Yellow, blue, and green flashes are the most common.
Violet, red, and orange are the rarest and most prized.