Physical
|
| Chemical Composition |
Silicon dioxide, or silica,
expressed by the formula SiO2.
The same composition as crystalline quartz, except that it usually contains larger amounts of impurities and may contain some amorphous silica in the form of opal.
|
| Crystallographic Character |
Cryptocrystalline; i.e. a compact, homogenous mass of submicroscopic doubly refractive crystals. A fibrous kind of crystalline structure (but not amorphous, for the most part). |
| Hardness |
6½ to 7 |
| Toughness |
Good; about the same as crystalline quartz. |
| Cleavage |
None |
| Fracture |
Conchoidal |
| Specific Gravity |
2.55 to 2.65 , normal: 2.60 |
| Streak |
White |
| Characteristic Inclusions |
Moss agate contains dendritic (treelike) inclusions of manganese oxide or sometimes iron oxide.
These inclusions, which resemble trees or plants, may be black, green, reddish, yellowish or brownish.
Segenite agate contains inclusions
of actionolite, hornblende, rutile
or other needlelike crystals.
|
Optical
|
| Degree of Transparency |
Semitransparent to opaque. There are no transparent varieties. |
| Luster |
Dull to waxy on rough surfaces; greasy to vitreous on polished stones. |
| Refractive Index |
1.535 - 1.539 (both readings are sometimes evident using a monochromatic light source) |
| Birefringence |
Weak |
| Optic Character |
Since chalcedony is a crystalline aggregate its optical characteristics cannot be determined by the usual gem testing methods. In the polariscope, translucent stones remain light during rotation. |
| Pleochroism |
None |
| Dispersion |
None |
| Phenomena |
Iris agate exhibits a display of rainbow colors (usually red, green
and blue) caused by diffraction of light from a multitude of very fine growth lines or bands. The so-called fire agate exhibits rainbow colors as a result of
interference of light from exceedingly thin layers of iron oxide
on the deeper layers of the stone.
Both of these are rare.
|
| X-Ray, Fluorescence |
All varieties are generally inert under both long and short wavelengths. Some, however, may fluoresce a weak
Green or blue, especially uranium bearing materials from Montana
and Australian chrysoprase. |
| Color-Filler Reaction |
Dyed except dyed green may show reddish. |
| Absorption Spectra |
Dyed green chalcedony may show vague chromium lines centered at about:7050, 6700 and 6450 A.U. |
