Hematite also spelled as haematite, is the mineral form of iron (III) oxide (Fe2O3). It forms a complete solid solution at temperatures above 950 °C (1,740 °F). Hematite is a mineral, colored black to steel or silver-gray, brown to reddish brown, or red. Huge deposits of hematite are found in banded iron formations. Gray hematite is typically found in places that can have still standing water or mineral hot springs. Hematite can also occur without water, however, usually as the result of volcanic activity.
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.