Black Onyx
- chalcedony (kal-SED-uh-nee) is microcrystalline or cryptocrystalline quartz; hexagonal (trigonal) crystal system; chemical composition SiO2
- semitransparent to opaque
- solid black, no banded, milky chalcedony with dyed treatment
Identification
- Optic Character: AGG
- 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
Enhancements
- 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
