Opal is a form of hydrous silicon oxide (silica, SiO2). Opal is never crystalline, it always occurs amorphously as veins, globules or crusts in its hostrock, translucent to clear and resinous to glassy or iridescent in appearance. When pure, opal is highly valued as a gemstone. It may occur in different colors, often combined in one specimen, but also in a single color. An example is the yellowish-orange variety "honey opal" here seen in unworked form on a white siliceous hostrock. Specimen from Juniper Ridge Mine, Oregon, USA, and actual width of sample is 160 mm