MAIN TYPES OF #OPAL

Crystal / Water Opal
Transparent to semi-transparent, with a clear background. Vivid and strong play-of-color

White / Light Opal
Translucent to semi-translucent, with play-of-color against a white body-color.

Black / Dark Opal
Translucent to opaque, with play-of-color against a black or dark grey background.

Fire Opal / Mexican Opal
Transparent to translucent, with red, orange, yellow, or brown body-color. May or may not show play-of-color.

Boulder Opal
Translucent to opaque, with play-of-color against a light to dark background and rock matrix.

Opal can be found around the world, with Australian #opal, specifically #black opal regarded as the most precious and valuable.

Australia
Solid (black, dark, light, crystal)
Boulder
Matrix​

Ethiopia
Solid (volcanic, hydrophanic)​

Mexico
Solid (fire opal / water opal)
Boulder (fire opal matrix) ​

Brazil
Solid (sedimentary light opal)​

Peru
Solid (blue and pink common opal)

#Opal is a hydrated amorphous form of silica (sio2·nh2o). When opal is formed, silica gel fills crevices in rock.

As water evaporates, the silica is deposited in the form of tiny spheres.

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.