Mookaite jasperJasper is an opaque variety of chalcedony (aka agate), formed of microcrystalline sil

Mookaite jasperJasper is an opaque variety of chalcedony (aka agate), formed of microcrystalline silica. There are many colours, patterns and types, born in a variety of ways. It has been used as a gem since time immemorial, and used to make items such as vases and seals, but this variety, only found in Western Australia, has a special history.The name comes from old French word for speckled stone, and while the current definition includes opacity, those of olden times used the word more widely, much as the ancient Chinese word for jade (yu) in practise meant any carveable stone. Minerals such as chrysoprase (another form of silica) were referred to under this label.The colours and patterns come from the original sediments or volcanic ash from which jasper forms and the impurities that result after diagenesis (the process of sediment transforming into rock) such as iron oxides, that give us the yellow and reds when oxidised, or greens when reduced. Some kind of heat is held to be necessary, whether from nearby magma or hydrothermal systems of hot fluids circulating in the Earth’s crust. Tree like dendritic patterns result from percolating minerals such as manganese oxide during or after formation.Mookaite has a particularly cool geological history. It started life as a radiolarian ooze, a fancy way of describing deep ocean bottom mud made of the opaline silica skeletons of these marine microorganisms (see http://tinyurl.com/omrd5r6). Its origin in the deep ocean allows it to be nearly all silica, since any carbonate skeletons will dissolve below a certain depth as the ocean chemistry changes. Eventually these sea bottom sediments turned into land, and were cemented into rock by silica precipitated from warm groundwater.LozImage credit: Craig Elliothttp://www.mindat.org/min-27597.htmlhttp://topgems.homestead.com/gemstone_information_mookaite.html -- source link
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