Electrical Conduction - Thermal And Other Tests

Hot Point

In the past, the heat produced by a match flame has been used as a semi-destructive test for detecting certain kinds of artificial coloration and for separating such materials as amber and plastics on the basis of the odor produced. Recently, however, a safer and more accurate method has been developed. It consists of the careful application of an electrical resistance heated metal point. When the switch is turned on, the sharp point acts as the resistance in the circuit and heats rapidly. At a dull-red heat, it is sufficiently hot for most testing purposes.

One of the useful tests that can be performed with this instrument is the identification of paraffin-treated turquois. When the point is held at a distance of about one millimeter from a stone treated in this manner, the heat liquefies the paraffin slightly. This reaction is readily apparent under magnification.

Identifying plastic-impregnated turquois by characteristic odor requires the point to be touched against the specimen very briefly while being held just under the nose. The strong, acid like odors of most plastics used for this purpose are usually obviously. to most testers.

This test also is valuable for separating amber and ivory from plastic imitations, for the characteristic odors of the substitutes contrast markedly with those of the natural materials.

In both of the above applications, the hot point must be touched to the stone very carefully and only for a brief instant. Although total destruction is unlikely, re-polishing may be necessary, unless it is applied to an area that is not visible when the stone is mounted and worn. Turquois decrepitates if it is heated to a high temperature, and amber burns. Even a light touch may leave a mark-on any stone being tested, so the need for exceptional caution should be obvious. Ivory, plastics and many other materials are disfigured quickly.

Another use of the hot-point technique is to detect emeralds, rubies, sapphires or other stores that have been "oiled" to conceal fractures and/or to improve color. If liquid is present in the fractures, the hot point may cause it to move visibly under magnification or even to be driven to the surface.

There are other occasional uses for the hot point. For example, it will detect the wax that is sometimes used to improve the appearance of jade carvings, and it will soften the pitch that cements the two portions of a black-opal doublet. However, since thin opal may be cracked by the concentrated heat, the point should be applied from the back.

Chemical Tests

There are few chemical tests, that may be employed to advantage in gem detection. Carbonates may be detected by the application of a drop of hydrochloric (muriatic) acid, for rapid effervescence of carbon dioxide is visible. This is useful for azurite, malachite, smithsonite, aragonite, calcite, coral, shell, ivory and several others. Aragonite is an important pearl constituent, and calcite is used for carvings, is dyed and sold as "Mexican jade," and is a major constituent in coral and shell. Thus, the "acid test" is a handy aid in the testing of carvings and opaque gems.

Another test in which acid is useful is in the detection of dye in black pearls. A cotton swab is dampened slightly in a dilute solution of hydrochloric acid (about fifty parts of water to one part of concentrated acid). When it is rubbed against a black pearl, a surface treatment is detected by a faint discoloration of the swab. The spot that was tested should be cleaned with a damp cloth, to remove any acid that remains.

Ether is sometimes helpful in separating amber from such natural, recent resins as copal and kauri gum. When ether is applied to these substances they swell, soften and become sticky, whereas amber is unaffected. Demmar resin, another material that may be mistaken for amber, is similarly affected by ethyl acetate.


The use of hardness points on a faceted transparent stone is an admission of defeat. It should not be needed to identify stones of this category, and it is unlikely to be of any value if it is used. Even when used with care, the ultimate destruction of a stone is not unusual. A fracture may originate at the scratch mark and continue across the entire stone. The glazier scratches sheet or plate glass to make it separate more easily; a scratch may have the same effect on a gemstone.

Opaque stones are less apt to break, and a small scratch on the back is concealed. Thus, hardness testing of opaque's, particularly in the rough, is more frequently employed by trained gemologists. Even so, it is used only when optical tests provide too little information to effect an identification. A hardness test is used ordinarily to distinguish a soapstone carving from serpentine, serpentine from nephrite, or another soft material from a substantial harder one.

The exceptional hardness of diamond is the cause of an irresistible urge by some gem-testers to check every diamond they identify, either by trying to virtually destroy it with a file or by testing it against a synthetic ruby or sapphire. The risk is greater than they realize, for breakage is not the only danger. If a visibly scratched stone is returned by an irate customer, he can make all kinds of demands. Even if the jeweler suffers no out of pocket loss, he may suffer a damaged reputation for carelessness and destruction of property.

The Conductometer

Diamonds having a blue body color in incandescent light are exceedingly rare in nature. Strongly fluorescent stones may have a faint bluish body color in daylight, but stones that are blue under any normal illumination are regarded as fancies. Some diamonds subjected to bombardment, or irradiation, by a stream of eleotrons become a slightly greenish blue. Since electrons have only a tiny fraction of the mass of neutrons (which penetrate throughout a diamond placed in a radioactive pile), the depth of penetration is only a fraction of a millimeter.

Diamonds made blue by electron irradiation may be distinguished readily from naturally blue stones, which have been found to be the rare type lib diamond. Type II diamonds are laminated, and are transparent to ultraviolet down to about 2250 A.U. Types IIa and Ilb are distinguished by the latter's strong blue phosphorescence to shortwave ultraviolet and its ability to conduct an electric current. The conductometer is a small instrument designed to quickly test a gem's ability to conduct electricity.

Loose stones are placed on a metal plate which acts as one pole. A metal post, called the ring holder and used for jewelry pieces and rings, acts as another pole. The moveable probe is touched to either loose diamonds or ring-mounted stones to see if they conduct. If the diamond conducts, a volt-meter registers the voltage. Only type IIb diamonds conduct electricity; most diamonds are insulators. Thus, blue diamonds that fail to conduct are colored by irradiation.

Color Filters

When the eye sees a colored stone, it is not capable of determining whether the color is caused by the absorption of more of all others except that one hue or by a combination of transmitted colors. Colors caused by one oxide as the coloring agent in a given hue may be distinctly different from those in the same hue caused by some other oxide. For example, iron oxide is responsible for the green color in a number of gemstones and chromium oxide in others. In some cases, the greens are indistinguishable to the eye. Using this information and examining the absorption spectra of different green gemstones with the spectroscope, Anderson and Payne, of the London Laboratory, devised the filter known in England as the Chelsea filter and in this country as the emerald filter. Because it is so inexpensive and in its way so simple to use, this little instrument is used in more jewelry stores than any other piece of gem-testing equipment.

This filter transmits readily a fairly narrow band in the yellow-green and another in the far-red portion of the spectrum. The green color imparted to emerald by chromic oxide is not within the readily transmitted band in the green, and so the secondary transmission peak is seen: a red color. Thus, emeralds turn red under the filter, whereas most natural green stones and imitations remain green. The better the color of an emerald, the redder it appears. Unfortunately, synthetic emerald also turns red. Also, demantoid and some green zircons appear pinkish or reddish when viewed through the filter. Occasional triplets are made with a green coloring that causes them to appear red, and some green plastic coated beads give the same reaction.

There are some other minor uses for the emerald filter. For example, the two tones of synthetic blue spinel that resemble sapphire and blue zircon appear red through it. This usually is true also of the aquamarine imitation. On the other hand, blue sapphire, zircon and aquamarine appear gray or green.

Anderson has also used an interesting combination of filters that he refers to as crossed filters. A beam of light is passed through a flask filled with copper-sulphate solution. This blue liquid cuts out the long-wave end of the spectrum, so that no red nor orange wavelengths and just a little yellow will pass. A stone placed in the path of a light beam passing through the flask is then viewed through a red gelatin filter, which cuts out that portion of the filter passed by the copper-sulphate solution. When viewed in this manner, ruby, synthetic ruby and red spinel take on a pleasing red color. It is clear that there is only one way to account for this: that these three stones fluoresce to a wavelength of light in the green-blue-violet portion of the spectrum. Actually, it is known that the fluorescent red color of ruby is excited to the greatest extent in the green portion of the spectrum, at approximately 5100 A.U.

Filters of various colors are used for other purposes in gem testing. The ultrathin coatings applied to diamonds at or near the girdle may be made to seem more in contrast against the background if a combination of filters is used. Orange and blue glass filters in combination serve fairly well. This type of coating is so thin and so confined in area that it is exceedingly difficult to detect, even under high magnification. To some microscopists, the filters seem to increase its visibility.

A variety of filters are used by different gemologists to sharpen white-light refractometer readings. Probably the most effective, because it produces a very narrow spectral transmission, is a red gelatin filter. Even a few thicknesses of red cellophane are fairly effective. Unfortunately the red wavelengths passed are far from the center of the spectrum, and the results are all high. The amount varies, but usually it is on the order of .005. An allowance can be made to adjust back to the usual sodium-light figure. The sharpening of the reading makes the trouble well worth while. Combined with a Polaroid plate, even stones with a very low birefringence may be proved to be doubly refractive without the expense of a monochromatic sodium light. It is possible to determine optic character and sign as well on most doubly-refractive gemstones.


Recently, B.W. Anderson of the London Laboratory developed a method for obtaining the relative degree of magnetic strength of a gem. The technique determines the gem's loss of weight on a diamond balance when it is attracted by a magnet field closely above it. Anderson suggested that the gem be placed on a light-weight, non magnetic pedestal, such as a cork, to raise it appreciably from the pan to eliminate the pull due to the metal of the pan. The previously weighed stone and the stone and cork pedestal together are weighed and noted. A small magnet is then held just above the stone, and the loss in apparent weight measured and recorded to arrive at a magnetic factor. Anderson used the formula:-

Pull= Weight loss x 100

to represent the "magnetism" of the stone and listed the values obtained for 15 stones.

Some of the Anderson's results indicate that one could differentiate between the following stones (the more magnetic of the two is listed first)

Also, this simple technique is of practical value in a preliminary separation of rough material.