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Download hereGENERAL DESCRIPTION
Opal is a hydrated variety of silica, resulting of a complex mixture of tridymite and/or hydrated cristobalite and/or amorphous silica. Thus, the opal should rather be regarded as a rock. But, in 2007, it was decided that it would keep its mineral status, for historical reasons. Thus, it is a semi-precious stone, known since the Antiquity and for this reason, it cannot be a rock.
The opal is formed in the basalts or volcanic tuffs weathered by hydrothermal fluids. Thanks to the phenomenon of pseudomorphosis, the opal can replace such foreign substances (for mineralogist) such as bones, wood or shells. This phenomenon corresponds to a mineralisation.
The colours palette for opal is vast: it extends from milky white (noble opal) to electric blue, while passing by the orange red one (opals known as of "fire opal"). The colourless variety names hyalite. One will distinguish the common opals (opals not showing iridescence) and the invaluable opals (opals showing iridescence). These iridescences are actually due to the light diffraction by layers of cristobalite spherulaes, perfectly arranged together. This arrangement is observable with a scanning electron microscope.
The opal is known as "amorphous" because its crystalline system does not present any element of symetry. Finer studies show rather than it is the complex arrangement of cristobalite balls that result in a pseudo absence of symetry elements. Recent studies in nanomineralogy show that opal contains many nanocrystals of cristobalite but not as "amorphous" as often reported in the past.
The opal deposits are numerous, but only a few produce invaluable opals: Slovakia, Austria, China, Germany, the Czech Republic, USA, Mexico (fire opals). The best iridescent opals are primarily originating in Australia: Queensland, New Wales of the South, Australia of the South, Tasmania...
In France, opals are rather widespread in some particular deposits of the following areas: Auvergne, Brittany, the Rhône-Alpes, Limousin, Centre and Provence-Alpes-Côte d’Azur.
The type is not definable because it acts of a described old species before the conditions of deposit of the types were not defined.
Did you know? Many varieties of opals are used in jewellery and are classified among the "fine stones". Being a hydrated mineral, the conservation of the opals implies that its place of conservation is not heated too much and that the samples are regularly humidified.
HISTORY : Name derived from the Sanscrit "upala" meaning a precious stone; name giving later "opalus" in Latin
CHEMICAL FORMULA : SiO2 nH2O
CRYSTAL SYSTEM : Amorphe
COLOR : White, blue, yellow, violett, red, black, irisations
DIAPHANIETY : Transparent to opaque
LUSTER : Vitreous to pearly
STREAK : White
MORPHOLOGIE : Amorphe
HARDNESS : 5,5-6,0
CHEMICAL CLASS: 1,99-2,25
DENSITY : IV - Oxides and hydroxides
GROUP : Moganite - opale series
STRUNZ CLASS BEFORE 2001 : 4/D.01-80
STRUNZ CLASS AFTER 2001 : 4.DA.10
