Opal is a hydrated amorphous form of silica. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt. Precious opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the black opals are the rarest, whereas white and greens are the most common.
Turquoise is an opaque, blue-to-green mineral that is a hydrated phosphate of copper and aluminum. It is rare and valuable in finer grades and has been prized as a gem and ornamental stone for thousands of years owing to its unique hue. The finest of turquoise reaches a maximum hardness of just under 6, or slightly more than window glass. The luster of turquoise is typically waxy to sub vitreous, and transparency is usually opaque, but may be semi translucent in thin sections. Color is as variable as the mineral's other properties, ranging from white to a powder blue to a sky blue, and from a blue-green to a yellowish green.