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SiO2 · nH2O
colourless, white, ...
5½ - 6½
A variety of Opal

Opal-AN is a term for amorphous opal with a glass-like structure (Graetsch, 1994). The "A" in the name stands for amorphous; the subscript "N" is to imply its structure is network-like similar to silica glass; however, it still contains about 3-8% water, both as molecular water and bound in silanole (SiOH) groups.

This form of opal occurs as hyalite in volcanic and pegmatite environments where the silica deposits from the gas phase (Flörke et al, 1973) and forms globular and irregular crusts. Because of its homogeneous internal structure, it does not show opalescence and is transparent if pure. The refractive index varies slightly with water content and lies between 1.44 and 1.46 (Graetsch, 1994).

Classification of Opal-AN

Physical Properties of Opal-AN

Diaphaneity (Transparency):
Transparent, Translucent
colourless, white, creme-coloured
no opalescence (play of colour)
Hardness (Mohs):
5½ - 6½
None Observed
2.0 - 2.2 g/cm3 (Measured)    
varies with water content

Chemical Properties of Opal-AN

SiO2 · nH2O
Elements listed in formula:

Occurrences of Opal-AN

Other Names for Opal-AN

Name in Other Languages:

Other Information

Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.

References for Opal-AN

Reference List:
Flörke, O.W., Jones, J.B., Segnit, E.R. (1973) The genesis of hyalite. Neues Jahrbuch der Mineralogie, Monatshefte, H.2, 82-89.

Graetsch, H. (1994) Structural characteristics of opaline and microcrystalline silica minerals. In: Reviews in Mineralogy, Volume 29, Silica - Physical behavior, geochemistry and materials applications. Mineralogical Society of America, Washington, D.C.

Internet Links for Opal-AN URL:
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The following Opal-AN specimens are currently listed for sale on

Localities for Opal-AN

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