Opal in Jasper
This seam of bright blue iridescent opal in jasper was found on the Barcoo river in Queensland, Australia. It is seen here under glass in the Minerals section of the Natural History Museum in South Kensington, London.
Opal is a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3-21% by weight, but is usually between 6-10%. Due to its amorphous property, 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, which means 'jewel', and later the Greek derivative opállios, which means 'to see a change in colour'.
There are two broad classes of opal: precious and common. Precious opal displays play-of-colour (iridescence); common opal does not. Play-of-colour is defined as "a pseudo-chromatic optical effect resulting in flashes of coloured light from certain minerals, as they are turned in white light." The exhibit seen above is a fine example of the iridescent version. Note that the way the natural light is falling on the exhibit has enhanced the effect.
The internal structure of precious opal causes it to diffract light, resulting in play-of-colour. Depending on the conditions in which it formed, opal may be transparent, translucent, or opaque, and the background colour may be white, black, or nearly any colour of the visual spectrum. Black opal is considered the rarest, while white, grey, and green opals are the most common.
Opal in Jasper
This seam of bright blue iridescent opal in jasper was found on the Barcoo river in Queensland, Australia. It is seen here under glass in the Minerals section of the Natural History Museum in South Kensington, London.
Opal is a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3-21% by weight, but is usually between 6-10%. Due to its amorphous property, 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, which means 'jewel', and later the Greek derivative opállios, which means 'to see a change in colour'.
There are two broad classes of opal: precious and common. Precious opal displays play-of-colour (iridescence); common opal does not. Play-of-colour is defined as "a pseudo-chromatic optical effect resulting in flashes of coloured light from certain minerals, as they are turned in white light." The exhibit seen above is a fine example of the iridescent version. Note that the way the natural light is falling on the exhibit has enhanced the effect.
The internal structure of precious opal causes it to diffract light, resulting in play-of-colour. Depending on the conditions in which it formed, opal may be transparent, translucent, or opaque, and the background colour may be white, black, or nearly any colour of the visual spectrum. Black opal is considered the rarest, while white, grey, and green opals are the most common.