Hyalite is a form of opal with a glassy and clear appearance which may exhibit an internal play of colors if natural inclusions are present. It is also called Muller's glass, water opal and jalite. The name Müller's glass derived from the name of its discoverer, Franz-Joseph Müller von Reichenstein.
Hyalite's Mohs hardness is 5.5 to 6 and has a specific gravity of 1.9 - 2.1. It has no planes of cleavage but fractures conchoidally, is clear or translucent and has a globular structure. Its luster is vitreous and its streak is white. Hyalite is an amorphous form of silica (SiO2) formed as a volcanic sublimate in volcanic or pegmatic rock and is therefore considered a mineraloid. It contains 3 - 8% water, either as a silanol group or in molecular form.
Opalescent hyalite is used in jewellery, and well-formed samples are of interest to collectors due to their unusual appearance, mode of formation and relative rarity. It is sometimes mistaken for resin opal or silica glass since they both may appear clear and globular, but it can be identified under ultraviolet light due to its bright green fluorescence.
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