The tests commonly used in gemstone identification are based upon definite, tangible instrument determinations; the refractometer and specific-gravity tests give definite numerical results, the dichroscope may show distinct colors, and polariscope determinations may be clear cut. Employing prescribed instruments and established methods a jeweler, after sufficient practice, can become skillful in securing satisfactory results with these instruments and methods.
By comparison, proficiency in identification of gemstones by means of their characteristic inclusions or imperfections involves a knowledge not quickly obtained by reading the printed page. To acquire skill in this method of identification, the jeweler must be thoroughly familiar with the subject of magnification; he must have a keen eye for accurate classification of inclusions or imperfections that involve, at times, the most fanciful shapes. Much more experience in the observation of gemstones will be required to achieve skill in this method.
Each time a jeweler makes a positive identification by the standard methods, he should examine the gem under high magnification In order to build up a working knowledge of the internal characteristics of gemstones.
LIGHTING. Ordinarily, a jeweler examines a gemstone with the light directed from behind the stone, with the result that inclusions appear as dark objects against a light background.
Dark-field illumination, in which the light is directed upon the stone from the side, is by far the best method of lighting for an examination of inclusions, since It not only enables the observer to locate imperfection more readily, but also aids identification of included crystals by revealing them as light objects against a dark background.
INCLUSIONS. This term is used in its broadest sense to include surface and internal fractures and cleavages, gas and liquid inclusions and crystal and other solid materials enclosed within the gem. In some gemstones the inclusions are sufficiently characteristic to permit an immediate identification of the stone.
Gemstones without internal flaw are not uncommon, especially in such gem species as diamond, beryl, topaz and quartz.
FRACTURE. Four gens may be identified with some degree of certainty on a basis of their fractures.
CHALCEDONY has the conchoidal fracture common to most gems, but the luster on the fractured surfaces is dull or waxy, not glasslike as in other gems.
HEMATITE fractures are characteristically splintery, resembling a break in wood.
TURQUOISE is often identified by its dull to waxy luster on small fracture surfaces, in contrast to the viterous luster of its glass imitation.
ZIRCON derives a characteristic appearance from its strong tendency to "pit'' or crumble at facet edges. Heat-treated zircons are especially subject to such pitting.
Most other gems display a conchoidal, or shell-like, fracture, with a vitreous luster on the fracture surface.
CLEAVAGE. Since few gems of importance are likely to show cleavage, straight cracks in a gem are important as clues to its identity; the angles between cleavage cracks may assist the jeweler to determine the system in which the gem crystallizes.
DIAMOND, the feldspar gems, spodumene and topaz are the important gemstones in which cleavage is likely to be observed.
Important Genuine Gemstones And Their Characteristic Inclusions
Corundum. Study the photomicrographs of corundum carefully. Usually, the experienced gemologist can identify the corundum family under the microscope immediately by means of several types of characteristic inclusions.
The crystal inclusions encountered in ruby and sapphire have the following characteristic appearances:
Needlelike inclusions, known as silk/consist of long crystals of rutile, straight and needlelike in appearance, and arranged in three sets of parallel threads that intersect each other at sixty degree angles. The three sets are all in planes at right angles to the c-axis ( in this case also the optic axis ). Rutile or hornblende needles in almandite are usually, but not always, coarser than those in corundum. They differ distinctly, in that only two sets (at 70° to one another) occur in the some plane. Needlelike inclusions in quartz are very short, usually occurring in small bundles with three directions at 60° and 120° to another in each grouping.
Included zircon crystals are characteristically surrounded by a halo of black fractures. Zircon, with its higher refractive index, stands out against the surrounding corundum as a bright point flight. The black halo of fractures around zircon crystals is thought to be caused by radioactive disintegration in the zircon.
Tiny spinel octahedra (eight-sided crystals that resemble two pyramids base to base) are found in corundum, especially in rubies from Burma and sapphires from Ceylon.
Other solid crystal inclusions that may be encountered in ruby and sapphire are:
Mica inclusions, six sided, colorless or brown.
Hematite slabs, brown or black. (often with a hexagonal outline).
Garnet in rounded grains
Rutile in coarse crystals.
Corundum crystals and grains with low relief
Many of these inclusions may be seen in rod garnets, especially the zircon crystals with halos, coarse rutile, hematite slabs and rounded grains.
The fingerprint inclusions take their name from interesting clouds of hollow inclusions filled with liquid and gas that form patterns resembling fingerprints around crystal inclusions. Though similar inclusion filled planes occur in other gems, the liquid inclusions rarely have the regular pattern common in ruby and sapphire. Fingerprints are rarely seen in garnet.
Siam rubies are characterized by fingerprint inclusions, black solid
inclusions and a lack of silk common to corundum from other localities.
Very prominent hexagonal growth and color zones are common in both ruby and sapphire in Burma rubies, however, a streaked and wavy color distribution is characteristic.
An effect created by repeated twinning constitutes in interesting phenomenon occasionally seen in corundum the only colored gem other than feldspar likely to show it. Straight parallel lines, more widely spaced than silk or color-zoning striae, extend all the way across the gem. When the twinned stone is placed in the dark (crossed Polaroid) position in the polariscope or under the polarizing microscope, the gem remains light in all positions and does not exhibit the usual four light and four dark positions of a doubly refractive gem during a three hundred and sixty-degree rotation. A second set of such lines may be present at right angles to the first.
TOURMALINE. Red tourmaline (rubellite) is typified by many internal fractures that are roughly parallel to the long axis of the crystals. The fractures are usually gas filled and give mirror-like reflections.
Green tourmaline seldom contains fractures parallel to the long axis of the crystal. It is characterized by long, irregular, threadlike liquid and gas inclusions, evenly distributed in abundance through-out the gem. Rubellite has these same capillary-size liquid inclusions, but seldom in the abundance common in green tourmaline have an appearance unlike other gems.
ALMANDITE. Under magnification almandite garnet is likely to be confused with ruby, since it sometimes contains grains of radioactive zircon as well as silk in a pattern that may appear similar to that found in ruby. There, however, the similarity ends.
Almandite garnet is frequently observed in which one can see two sets of needlelike inclusions intersecting at angles of 70° and 110°. The angle of 70° is close to, and might be mistaken for, a 60° angle on casual inspection. Since there are three such paired sets of inclusions in some almandite, there are certain directions along which a three-fold grouping of inclusions similar to that of corundum may seem to be present; however, only two directions are ever found in the same plane in garnet.
The silk in almandite is coarser, shorter and usually less abundant than its counterpart in corundum. Evenly distributed, small colorless grains in great abundance that are often doubly refractive in the singly refractive garnet, together with the stubby silk, suggest almandite garnet.
GROSSULARITE. Grossularite garnet usually contains short, stubby, rounded prisms (probably of diopside) in quantity. A characteristic peculiar to the hessonite variety of grossularite is a swirled heat-wave-over-hot-pavement effect that gives the observer the impression that it is impossible to properly focus his microscope on the interior of the gem.
ANDRADITE. The demantoid variety of andradite garnet exhibits brown inclusions similar to very fine silk, but in characteristic curved and radiating arrangements, which identifies it at once.
PYROPE. Pyrope garnet has an internal appearance similar to that of almandite, but often with large rounded crystal grains of very low relief.
EMERALD. Emerald, one of the gemstones most easily identified by its imperfections, contains not only many crystal inclusions, but also three-phase inclusions- irregular spaces filled with solid, liquid and gaseous matter.
Emeralds have such a variety of inclusions that an experienced tester may be able to gain a good idea of source by characteristic inclusions. For example : three-phase inclusions with a square or rectangular crystal phase typify Colombian emerald; tremolite needles in a rich green, small stone suggest Sandawana; and pyrite crystals suggest Colombian origin. The characteristically shaped brass-yellow cubic crystals of pyrite are often seen in emerald, and because they appear black in transmitted light, are usually referred to as "carbon".
A badly fractured appearance is very common in emerald under magnification. Calcite inclusions along fractures are common in emeralds from Colombia, often imparting a roiled appearance. The emerald from most of the localities of the world have inclusions which are characteristic of that area. If one has access to emeralds from known localities, a gemologist can often be relatively sure of their source.
AQUAMARINE. Though aquamarine is often free from inclusions, it may show characteristic brown, iron-oxide inclusions and tiny, parallel liquid-filled spaces.
spinel- spinel's characteristic inclusions are usually formed by tiny enclosed octahedral (eight-sided) crystals of spinel. The included crystals are found both scattered in a random distribution, and in layers of many crystals. The layers of crystals are sometimes parallel to octahedral faces of the spinel where they formed as the crystal grew, but more often these are distributed along irregular fractures.
TOPAZ. Topaz is more likely to be free from inclusions than almost any other important gem. Its characteristic inclusions are irregular, often fairly large liquid and gas-filled spaces that may contain two or more non miscible liquids separated by a clear dividing line. The easy cleavage parallel to the base of the orthorhombic topaz crystal is sometimes shown in the cut gem by straight feathers. Clear signs of cleavage serve to separate topaz from most of the gems with which it is confused.
High property zircon does not have distinctive features likely to be encountered in a majority of stones examined. However, the sum of common features provides a valuable indication of identity.
The high birefringence of all but green or the very rare orange metamict zircon results in a strong doubling of the opposite facets in zircon of any other color. The junction of two facets appears to be two lines when the microscope is focused through the gem onto the pavilion facets. Similarly, inclusions in all but green zircon appear doubled in any direction at more than a small angle to the axis of single refraction.
White zircon often has many inclusions so tiny that they cannot be resolved individually, but give a total effect referred to as "cottony."
Zircon, unlike diamond, appears cloudy, rather than clear or sharply transparent.
Occasionally, zircon contains flat planes of wormlike inclusions, roughly circular in contrast to the angular-patterned liquid inclusions of the corundum fingerprint.
Low-property zircon, usually green, is characterized by the presence of strong parallel zoning. All low-property zircons examined have exhibited this characteristic.
The species of quartz has more gem varieties than any other mineral. The crystalline varieties amethyst, citrine, rock crystal and smoky quartz are characterized by inclusions of negative crystals in the usual hexagonal crystal form of quartz.
Amethyst and the citrine resulting from heat treatment of amethyst often show cloudy white inclusions that appear as white stripes in a plane. They resemble a soap scum on hard water. A very sculptured fracture surface with sub vitreous luster is also characteristic. However, crystalline quartz is often flawless.
Gryptocrystalline quartz (chalcedony) has no inclusion typical of all varieties. The dendritic arrangement of manganese oxide in moss agate is characteristic of that variety.
The inclusions that cause a star in quartz are unlike those that produce asteriated ruby and sapphire. The needlelike inclusions are very short and occurs in small "bundles" distributed at random throughout the gem.
PERIDOT. Tiny black metallic inclusions, surrounded by a small fingerprint pattern of liquid inclusions, characterize peridot when present, but many peridots do not contain such inclusions. Peridot is strongly birefringent.
DIAMOND. Very frequently seen are included crystals of diamond (likely to be confused with black carbon inclusions unless viewed properly; i.e., by dark-field illuminiation). Elongated, four-sided prismatic crystals that may be peridot, pyrope garnet or chrome diopside are not uncommon.
The three keys to a diamond identification under magnification
are the unique appearance of the surface of a bruted girdle on a
brilliant or a marquise, a grooved or trigon-studded natural (an
original crystal surface) and cleavages.