The Varieties of Silica:
Quartz, Chalcedony & Opal

Quartz, chalcedony and opal are all forms of silica (SiO2). The difference is that quartz has visible crystals whereas chalcedony crystals can only be seen under a microscope and opal has no crystals at all.

Quartz is, by far, the most common mineral on earth. 

There are hundreds of varieties of quartz, chalcedony and opal, mostly distinguished by their color.

 

Some of the more common varieties of quartz are:

 

Quartz specimens now available (at eBay):

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60g Natural Green Skeletal Elestial Quartz Cluster Specimen,Inner Mongolia

AU $0.01 (0 Bids)
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28.5g Natural Green Skeletal Elestial Quartz Cluster Specimen,Inner Mongolia

AU $19.99
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Some of the common varieties of chalcedony are:

   

Chalcedony specimens now available (at eBay):

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16.5g Blue Chalcedony Stalactite Crystal Mineral Specimen Zeolite India C510

AU $7.99 (0 Bids)
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79.30gr 100% Natural Purple Botryoidal Chalcedony Grape Agate

AU $35.00 (0 Bids)
End Date: Tuesday Oct-24-2017 16:37:54 EST
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43g Apophyllite Cubes on Grey Chalcedony Mineral Specimen Zeolite India AC420

AU $9.99
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CHALCEDONY - The middle of Nowhere, New Mexico 9x5.2x1.6cm Short Wave NICE

AU $42.00
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Bigstargarstone RED BALLS FLOWER CHALCEDONY ON MATRIX MINERALS

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Bigstargarstone OKNITE BALL ON CHALCEDONY BASE MINERALS

AU $9.99 (0 Bids)
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Bigstargarstone OKNITE BALL ON CHALCEDONY MATRIX MATRIX

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Some opal displays different colors when viewed from different directions. Any opal which exhibits this play of colors is called "precious opal"; opal which does not exhibit a play of colors is called "common opal". Some of the common varieties of opal are:

Opal specimens now available (at eBay):

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Large white/blue raw/natural opal specimen 497 grams

AU $120.00 (0 Bids)
End Date: Saturday Oct-21-2017 13:30:27 EST
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44 pieces 70+crt. coober pedy + lightning ridge opal

AU $100.00 (0 Bids)
End Date: Friday Oct-20-2017 22:27:47 EST
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Yowah Nut Material Australian Boulder Opal Unpolished Gemstone Specimen RP914

AU $214.50
End Date: Thursday Oct-26-2017 12:18:22 EST
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Beautiful West Australian green opal with magnesite mineral slab with stand

AU $105.00
End Date: Tuesday Oct-31-2017 16:23:04 EST
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