OPAL

Opal is a hydrated amorphous variant of silica (SiO2·nH2O), with its water content varying from 3% to 21% by weight, typically falling between 6% and 10%. Due to its amorphous structure, it is categorized as a mineraloid, in contrast to crystalline silica, which is classified as a mineral. Opal forms at relatively low temperatures and can be found in the fissures of various rock types, most frequently associated with limonite, sandstone, rhyolite, marl, and basalt.

The term "opal" is thought to originate from the Sanskrit word "upala" (उपल), meaning 'jewel', which later evolved into the Greek term "opállios" (ὀπάλλιος).

Opal can be divided into two main categories: precious and common. Precious opal exhibits a play-of-color (iridescence), while common opal does not. Play-of-color refers to a pseudo chromatic optical effect that produces flashes of colored light from certain minerals when exposed to white light. The internal structure of precious opal allows it to diffract light, creating this effect. Depending on its formation conditions, opal can be transparent, translucent, or opaque, with background colors ranging from white to black and nearly every hue in the visual spectrum. Among these, black opal is regarded as the rarest, whereas white, gray, and green opals are the most prevalent.