Mineraloid in the context of Jet (gemstone)

In geology, a rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition, and the way in which it is formed. Rocks form the Earth's outer solid layer, the crust, and most of its interior, except for the liquid outer core and pockets of magma in the asthenosphere. The study of rocks involves multiple subdisciplines of geology, including petrology and mineralogy. It may be limited to rocks found on Earth, or it may include planetary geology that studies the rocks of other celestial objects.

Rocks are usually grouped into three main groups: igneous rocks, sedimentary rocks and metamorphic rocks. Igneous rocks are formed when magma cools in the Earth's crust, or lava cools on the ground surface or the seabed. Sedimentary rocks are formed by diagenesis and lithification of sediments, which in turn are formed by the weathering, transport, and deposition of existing rocks. Metamorphic rocks are formed when existing rocks are subjected to such high pressures and temperatures that they are transformed without significant melting.

Opal is a hydrated amorphous form of silica (SiO₂·nH₂O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous (chemical) physical structure, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered minerals. 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.

The name opal is believed to be derived from the Sanskrit word upala (उपल), which means 'jewel', and later the Greek derivative opállios (ὀπάλλιος).

Fiorite is a hydrated silica mineraloid, a form of opal, found in cavities in volcanic tuff. It is a globular, botryoidal, or stalactic concretionary form of opal. The mineraloid has a pearly lustre and forms botryoidal masses. Named after Santa Fiora, Italy, fiorite is used as a gemstone.

Opalescence or play of color is an optical phenomenon associated with the mineraloid gemstone opal, a hydrated silicon dioxide. This effect appears as a milky, translucent glow that changes with the angle of light, often creating a soft, pearly sheen that can display various colors or hues. Opalescence can be seen in materials like certain minerals, glass, and even fluids.

Chrysocolla (/ˌkrɪsəˈkɒlə/ KRIS-ə-KOL-ə) is a hydrous copper phyllosilicate mineral and mineraloid with the formula Cu
_{2 – x}Al
_x(H
₂Si
₂O
₅)(OH)
₄⋅nH
₂O (x < 1) or (Cu, Al)
₂H
₂Si
₂O
₅(OH)
₄⋅nH
₂O).

The structure of the mineral has been questioned, as a 2006 spectrographic study suggest material identified as chrysocolla may be a mixture of the copper hydroxide spertiniite and chalcedony.