Home   |  Gemstone Guide   |  Semi-Precious Gem Guide   |  Organic Gem Guide


: Chalcedony



Chalcedony used in Jewelry


Chalcedony Crystals


Source: Occuring worldwide


Chalcedony is a fiberous, translucent to opaque, compact microcrystalline variety of quartz that occurs in white, gray, and grayish-blue hues. Chalcedony is formed by silica-rich water percolating through cavities and fissures in volcanic rock. Chalcedony is a relatively porous material that is easily dyed to alter or enhance the color. Chalcedony that has distinct banding is called "agate." The name "chalcedony" (calcŽdoine French, chalzedon, Kalzedon German, Piedra de luna Spanish or Italian) is derived from the ancient sea port of Khalkedon, at the crossroads of Europe and Asia in Constantinople (now Istanbul, Turkey).



Pure chalcedony is white, but there are several varieties of chalcedony that go by different names depending on coloration. Green chalcedony gets its color from trace amounts of nickel oxide, and goes by the name 'chrysoprase,' while red varieties of chalcedony goes by the name 'carnelian' or 'sard.' Black chalcedony is referred to as onyx, and brown chalcedony is referred to as jasper. Banded varieties are referred to as agate.


Chalcedony Earrings by Michael Sherman

Chalcedony Earrings by Michael Sherman

   Chalcedony Agate Geode

Chalcedony Agate Geode


Both agate and chalcedony are typically cut into a cabochon or beads. Chalcedonies are found worldwide. Chalcedony was perhaps one of the first materials used by early man to form tools and arrowheads, due to its durability and abundance, and flint used as a fire-starting tool, is a variety of chalcedony.



Chalcedony is crystallographically identical to the quartz, and belongs to the tectosilicate family of minerals. Chalcedony forms in the hexagonal/trigonal crystal system with a microcrystalline crystal habit.


Chalcedony (Microcrystalline Quartz) Chemistry & Physical Properties

Crystal System rhombohedral microcrystalline
Crystal Habit cryptocrystalline
Specific gravity (SG) 2.58 - 2.61
Mohs Hardness Scale 7
Toughness good
Fracture conchoidal, uneven, splintery
Cleavage none
Streak white
Chemical Composition SiO2 (Quartz)

Chalcedony (Microcrystalline Quartz) Optical Properties

Optical Properties singly refractive
Refractive Index 1.530 - 1.550
Birefringence +0.009
Pleochroism none
Surface Luster dull, greasy, silky, waxy
Diaphaneity translucent, subtranslucent
Gem Color blue, brown, gray, green, purple, white


Bloodstone

Bloodstone (also used for Hematite) is a bluish (bluestone) to greenish variety of chalcedony that is flecked with spots of red color from the presence of iron oxide. The red spots can take on the appearance of splattered blood, thought to be the blood of Christ during the Middle Ages. Bloodstone occurs in Australia, Brazil, China, India, and the United States.





Gemology Books
Gemstone Books




Bibliography and Reference on Chalcedony


1. Judith Crowe, The Jeweler's Directory of Gemstones . DK Publishing.

2. A.C. Akhavan, Twinning in Quartz Crystals www.quartzpage.de

3. GIA, Characteristics of Citrine, Amethyst & Smoky Quartz www.gia.edu

4. Renee Newman, Gemstone Buying Guide . International Jewelry Publications; 2nd edition

5. Antoinette L . Matlins, Antonio C. Bonanno, Gem Identification Made Easy . Gemstone Press




Gem Home   |  Semi-Precious Gem Guide

  

Copyright © 2012 AllAboutGemstones.com. All rights reserved.

  
  
Gemstone Books on Agate