Monthly Archive for September, 2008
Undoubtedly, the quickest and easiest of the reliable determinations is using a petro-graphic microscope, or a refractometer, to determine the refractive index. A trained micro-scopist can very rapidly differentiate between the varieties of jadeite, between jadeite and nephrite, and between jade and many of the jade-like minerals. Best of all, it takes only a few tiny grains of material, which can be removed easily from some inconspicuous place on a carving. The major drawback is that it requires an operator trained in the techniques of optical crystallography, as well as an expensive piece of equipment.
The most precise jade identification involves the X-ray diffraction powder method. This, too, requires only a very tiny amount of sample. A narrow X-ray beam is passed through the powdered mineral, which diffracts it into a pattern of lines. The pattern is recorded on photographic film, which can then be compared with a standard file of films containing patterns of known nephrite, jadeite, and other minerals. This kind of identification is fundamental because it depends on the fact that every crystalline mineral yields a characteristic pattern stemming from differences in internal structure. More info for diamond rings can be find here 1ct diamond engagement ring
A quick survey of obvious and not-so-obvious jade substitutes—many used innocently and with no intention of fraud—shows the magnitude of the identification problem. Only a very few substances are difficult to distinguish from jade. Jade-like glass, plastics, soapstone, and softer varieties of serpentine are easily separated from true jades because they are scratched readily by a good penknife. Jade carvings from ancient tombs will also scratch because their surfaces have become softened through centuries of exposure to the chemicals of decomposing bodies and soil solutions. Very old, archaic jade pieces may suffer the same alteration. If there is any doubt about these, they are worth an X-ray determination of the powdery surface material, which will show that they are still nephrite. Since jadeite was unknown in China until relatively modern times, all the ancient and altered jade will be nephrite.
Of the harder jade substitutes, the most troublesome to identify are bowenite; a green, fine-grained variety of idocrase known as cali-fornite; some fine-grained pieces of zoisite and diopside, and a few other related silicate minerals. For some, a determination of specific gravity is sufficient evidence. This, coupled with a reading of refractive index, is usually enough. Except in obvious cases, these silicate minerals are best left to trained mineralogists and gemologists for proper identification.
Jadeite is one of a group of minerals known as pyroxenes. Basically it is a sodium aluminum silicate. One variety of it found in Central America has an intermediate composition between jadeite and diopside—a calcium magnesium silicate; it is usually referred to as diopside-jadeite. Still another variety, called chloromelanite, with a strong, dark-green-to-black color, has a composition intermediate between jadeite, diopside, and acmite—a sodium iron silicate. Whether the sample is jadeite, diopside-jadeite, or chloromelaniie, all of these are unquestionably identifiable as pyroxenes and are close enough to jadeite to be recognized as merely slightly variant kinds of the same species.
The crystals of jadeite are not fibrous or matted, as are those of nephrite. However, they are complexly interlocked so as to have a compact, granular appearance. Jadeite, because of this difference, is not as tough as nephrite, but at 7 it is somewhat harder on the Mohs scale. It is measurably denser, too, than nephrite with a specific gravity varying from 3.30 to 3.36. Colors of jadeite tend to parallel those of nephrite, except that a pure-white or near-white jade is almost surely jadeite, as is a vivid emerald green 3 stone ring . Also reserved to jadeite are blue-greens, shades of lavender, and near red.
The challenge to the present-day jade carver and collector is an old one: how can jadeite and nephrite be distinguished from each other and from very similar substitutes? Most of the folklore of jade identification is worthless. For example, jade carvings do feel cool to the touch, as we are told, but so do many other stones. Simple tests can be applied, and long years of experience with jade can be used to make visual identification with a high percentage of success. In difficult cases, even among the experts, simple tests may fail miserably. Of course, there are more sophisticated methods available to the experts using expensive technical equipment and procedures that are just about foolproof.
Hardness is not a good test. Nephrite is 6 to 6i/2, and jadeite 61/2 to 7. Then, too, there is a variety of good green serpentine, called bow-enite, which looks very much like nephrite and has a hardness of 6. These hardnesses are all too close for certainty. The old trick of making a small, inconspicuous scratch with a penknife will not distinguish jadeite from nephrite, and will not eliminate bowenite or other hard species. It is useful for separating out the softer varieties of serpentine, soapstone, etc. Specific-gravity determinations are good, in that they do not require any mutilation of a carving.
The entire piece can be measured. Fortunately, too, jadeite has a specific gravity of 3.3 to 3.5, and nephrite of 2.96 to 3.10. The gap is sufficient to measure and distinguish one from another. But once again, bowenite, with a specific gravity just under 3.0, confuses nephrite identifications. Visual examination sometimes helps, because the expert soon comes to recognize a certain crystallinity in jadeite that is lacking in the other stones. Nephrite and serpentine have an amorphous internal appearance. A polished surface on nephrite seems to look more oily than glassy.