Chalcedony - Encyclopedia

    Class : Silicate
    Subclass : Tectosilicate
    Crystal system : Trigonal
    Chemistry : SiO2
    Rarity : Really common


The chalcedony is a microcrystalline to cryptocrystalline  variety of quartz which contains 0.5 to 2% water. It takes various names depending on its appearance and color : agate or onyx if the structure is banded with alternating colors, carnelian if it is orange-red color, sardoine if it is brown, chrysoprase if it is green and colored by nickel, mtorolite if it is green and colored by chromium, heliotrope if it is green with red spots. Specimens do not fall into these categories of colors and structures are called "jaspers". The chalcedony never show visible crystals, they often are hummocky fibrous masses sometimes covered with quartz crystals. It is a low-temperature hydrothermal variety which is widely used today in the ornamentation, sculpture ornaments and art objects. Critically, it is used for making mortar grinders necessary to achieve rock powders for laboratory analysis. Formerly, Neolithic men have used it to make tools.

Yellow chalcedony on lussatite from Mine des Rois, Dallet, France
Stalactitic chalcedony from Nasik, Maharashtra, India
Purple grape chalcedony from Mamuju, Sulawesi Barat, Indonesia
Agatized cerithium fossil from Assa, Morocco

Chalcedony in the World

The best deposits are located in South America. In Brazil and Uruguay, we find very large specimens used extensively in Asian countries as a material for sculpture (see photo in margin). In Germany, agate deposits from Idar-Oberstein are the most important in Europe causing a very important lapidary activity today. Equally interesting deposits from Eastern Europe which have fabulous blue to red multicolored banded agate. Some U.S. turquoise blue chalcedony colored by minerals of copper are also fantastic (especially Inspiration Mine in Arizona). Since 2015 in Indonesia, purple natural grape chalcedony clusters, sometimes of metric size, are extracted.
39.20 ct agate cabochon
21.72 ct blue lace agate from Namibia
45.00 ct crazy lace agate from Mexico
25.35 ct onyx (treated) from Brazil

Chalcedony in France

Chalcedony in France is very present. Among the most beautiful deposits, we mention the Etelon (photo in margin) and Châtelperron in Allier. Beautiful bluish chalcedony drops were also harvested once in asphalt mines area of Clermont-Ferrand in Puy-de-Dome. It may be noted also for its large hummocky specimens Aquitaine department and the Massif de l'Estérel in the South of France, famous for its agate and quartz nodules.

Twinning

Chalcedony is a microcrystalline variety of quartz, by definition it presents no twin. One feature to highlight, is the possibility of rotation of quartz fibers around their axis of elongation during growth. This phenomenon is typical of the chalcedony "flowers" from Mine des Rois of Dallet in Puy-de-Dôme, it generates regular and repeated extinction bands if observed under microscope with polarized and analized light (photo in margin).

39.20 ct heated carnelian from India
13.96 ct chrysoprase from Brazil
24.40 ct Noreena jasper from Australia
43.70 ct orbicular jasper from Madagascar

Special crystallizations

Chalcedony can quite easily replace some fossils or fill the cavities left by some fossils gastropod, corals, plants, etc.... It is called "agatized fossils". Among the most beautiful transformations, it is necessary to note the gastropods from Betul in India (photo on the right), transformed by fillings in agate, sometimes dendritic with gray, brown, white or bluish colors... The Eocene fossil shells of the sector of Assa in Morocco are also sometimes replaced by orange chalcedony and are surprising and very aesthetic collector's items.

Fakes and scams

Among the classic scams there Chinese brown-orange grapes, sometimes sold to natural when they are in fact carved, the photo in margin presents specimen sold on eBay for totally natural.

Chalcedony can undergo various treatments, the most common is dying. The chalcedony are immersed in an acid bath that will increase porosity of the mineral and then they are dyed in various colors, often quite "flashy". The fluorescent pink and green agates encountered on exhibitions today are all dyed (photo below) and onyx (black and white) too. Unfortunately, these treatments are almost never specified... These dyes are generally highly fluorescent under UV light and a cotton swab soaked in acetone can leave dyes residues.

Finally, some chalcedony can be heated to change their colors, most often to give carnelian.



Hardness : 6,5 to 7
Density : 2,6
Fracture : Conchoidale to sub-conchoidale
Trace : White




TP : Transparent to opaque
RI : 1,54 to 1,55
Birefringence :  -
Optical character : Uniaxial +
Pleochroism : None
Fluorescence : White to green


Solubility : Hydrofluoric acid

Magnetism : None
Radioactivity : None