Test and Identification of Chalcedony


Chalcedonic quartz, in one form or another, can be confused with almost every translucent to opaque gemstone. Nephrite, jadeite, malachite, lapis Lazuli, moonstone, amazonite, turquoise, shell and many other materials are closely duplicated in appearance by one or more varieties of natural or dyed chalcedony.

Green chrysoprase and green-dyed chalcedony can be separated from amazonite by the appearance of minute fractures along the girdle edge. Amazonite displays cleavage, whereas chalcedony shows only its characteristic dull to waxy conchoidal fracture. In addition, amazonite has a characteristics appearance; a sort of shiny reflection that appears to be just beneath the surface, accompanied by a grid like color distribution. Chalcedony, on the other hand, is usually rather evenly colored and lacks both of the characteristics mentioned for amazonite. They can also be distinguished by a careful spot reading on the refractometer, for amazonite has slightly lower indices. A heavy liquid set at 2.57 or 2.58 also separates the two stones.

Chrysocolla chalcedony, which in its finest quality bears a close resemblance to high-quality turquoise, can be distinguished there from by physical properties. Chrysocolla chalcedony is considerably less dense (2.60 to 2.75 plus) and has a lower R.I. (1.535 compared to 1.61). In appearance they are almost identical, sharing the translucency expected of the finest turquoise. It is impossible to distinguish between these two minerals on the basis of fracture or fracture luster, since both have a conchoidal fracture and a dull to waxy luster on fracture surfaces.

The mineral chrysocolla, which is distinct from chrysocolla chalcedony, has a blue enamel like appearance, in a color similar to that of turquoise, but it is soft (2-4) and crumbles easily. The refractive indices have been reported from about 1.4 to nearly 1.6, but 1.464 to 1.57 is usual. The S.G. is 2.2 to 2.4. Really, its only function in the gem field is to act as a coloring agent for chalcedony.

Dyed jasper made to imitate lapis lazuli is seldom a highly effective imitation on the close inspection. True lapis usually contains flecks of pyrite, whereas the dyed jasper does not. In addition, the jasper will give a characteristic chalcedony R.I. of approximately 1.535, as opposed to approximately 1.50 for lapis.

Jadeite and nephrite are distinguished from chalcedony readily by either refractive index or specific gravity test, since both jades have much higher property values.

Although dyed green agate may resemble malachite superficially, the very high S.G. and R.I. of malachite serve to separate it readily. To one who is familiar with the appearance of both materials, there is little resemblance. Malachite often displays a radial fibrous structure that is never obvious in agate. The two colors in malachite are very dark green and a lighter green, whereas one color in agate is usually white or almost white.

One of the key characteristics of chalcedony in its translucent to semi translucent forms is its behavior in the polariscope; since it is composed of a multitude of minute crystals with a random orientation, it remains tight in all positions in this instrument, rather than changing from light to dark. Moonstone, for example, shows extinction upon rotation, and in some positions exhibits an interference figure. Another characteristic of some translucent chalcedony is the fact that in certain lights it is possible to see a rather shadowy structure, which would suggest that the material is composed of rounded "blobs" of a fraction of a millimeter to about one centimeter is size.

Most stained chalcedony has a very intense color that is not characteristic of the natural. With the exception of chrysocolla quartz, the color intensity of unstained material is rather subdued. Moreover, the color of such attractive atones as chrysoprase is usually some what mottled, in contrast to the solid, even green of the more intensely colored stained green chalcedony.

Chalcedony is often imitated in glass. Usually, a separation can be effected by observing the luster of fracture surfaces; that of glass is vitreous, as opposed to the dull to waxy luster of chalcedony. In addition, semi transparent to translucent glass often shows bubbles and swirl marks close to the surface. The dendritic patterns in glass imitations of moss agate are much too uniform for this substitute to be  confused with the natural stone.

Shell cameos are sometimes confused with stone cameos, especially those carved from carnelian and white-layered agate or onyx. These may be distinguished readily by applying a tiny drop of hydrochloric acid, for the shell effervesces strongly and chalcedony fails to react. Shell's low (3-4) hardness is also easily detected.

Opal without play of color may closely resemble chalcedony of comparable color. A spot R.I. reading shows about 1.45 for opal and 1.53 for chalcedony. Translucent opal may well remain light in all positions in the polariscope; therefore, that instrument does not assist in this separation, unless the stone remains dark. S.G. differs widely, also.

 

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