Chalcedony – Black Onyx


• Chalcedony (kal-SED-uh-nee) is microcrystalline or cryptocrystalline quartz; hexagonal (trigonal) crystal system; chemical composition SiO2
• Semitransparent to opaque
• Black, no banded, some with dyed treatment from natural grey agate


• Optic Character: AGG
ž• Refractive Index: 1.535-1.539; 1.53 or 1.54 spot reading
• Birefringence: usually not detectable, but may show .004
• Pleochroism: none
• Ultraviolet Fluorescence: generally inert (LW and SW); some may fluoresce a yellowish green (LW and SW)
• Absorption Spectra: dyed green – vague lines around 645 and 670 nm
• Specific Gravity: 2.60 (+.10,-.05)
• Polish Luster: greasy to vitreous
• Fracture: conchoidal; sometimes granular
• Luster: dull to waxy
• Cleavage: none
• Identifying Characteristics: dyed green and dyed blue appear red or pinkish through Chelsea filter
• Crystal Habit: aggregate, often in botryoidal forms
• Hardness:6 ½  to 7
• Toughness: good


• Method: dyeing
• Effect: chalcedony is dyed to practically all colors for various uses; the bands in agate and Onyx accept dye differently so that a green dye, for example, may impart color to some layers and leave others white.
• Prevalence: common
• Detection: appearance (color often looks unnatural), spectrum, Chelsea filter reaction, SG (dyed green often lower (2.60 or less) than chrysoprase)
• Stability: generally stable
• Special Care Instructions: none
• Sources: Brazil, Uruguay.

Stone information is reference from:

• Gem Reference Guide, Published by GIA. ISBN 0-87311-019-6
• Gemstones of the World, Published by Sterling Publishing Co., Inc NY, ISBN 1-4027-4016-6
• Encyclopedia of GEMSTONE, Published by ISHINSHA CO., LTD. ISBN 978-986-251-831-1
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