Opal

silicates

1.90 to 2.20 g/cm3

amorphous

lack

SiO2·nH2O

colourless, white, yellow, blue, red, brown, green, gray, black

5 to 6.5 on the Mohs scale

glassy

Opal is formed as a result of the activity of water in the earth's crust; product of low-temperature hydrothermal, metasomatic and weathering processes; it is found in igneous rocks and sedimentary rocks, in the sediments of geysers and hot springs, it is a component of many sedimentary rocks; its main place of occurrence is Australia (most precious opals are mined there and reach the highest values), but it is also found in the USA, Slovakia, Mexico, Russia and South Africa; in Poland, only base opals can be found, mainly in Lower Silesia, near Zawichost and Chełm

It is a quite common mineral, but some of its varieties are considered noble and very rare; occurs in clusters, spherical, kidney-shaped, forms crusts, veins, encrustations, concretions and spatial skeletons (rocks), creates pseudomorphs; does not form crystals, is solidified colloidal silica with water content ranging from 3 to 10%; characterized by glassy gloss, lack of cleavage and conchoidal or uneven fracture; it has its various varieties, such as natural opal, black opal, fire opal, noble opal, wood opal, sandstone opal, moss opal, water opal, fiorite and hyalite; it is used in jewellery to produce precious jewellery, its noble varieties are highly valued collector's stones

Owes its name to the Greek word opallios, meaning "to see change", which is related to its very diverse colour palette; in ancient times, opals were very valuable and expensive stones imported to Greece and Rome from India for the governors of these countries; opal is the official gemstone of Australia (opals mined there are the most expensive); Australia extracts over 95% of the world's precious opals for use in jewellery; indigenous Australian myths often link the formation of opals with rainbows, which is why this stone is called the rainbow stone; in one myth, an ancestral being came to earth through a rainbow, and when the rainbow touched the earth, all the pebbles and rocks nearby began to spark and turned into opal; opals are very sensitive to temperature, which may cause water to evaporate from it and therefore may lose its visual properties, but treating it with water again will reverse this process, although the mineral will not always regain its original colour palette; the name of this mineral comes from the name opalization, which is a phenomenon in which the colour changes with a change in the viewing angle