To identify a gemstone there are a number of very accurate tests which a jeweler – gemologist can make use of. Each gem species has characteristic physical and optical properties which are constant with in narrow limits and to a certain extent can be objectively determined. However, a basic fact which must never be forgotten, is the importance of a visual identification. An unknown stone can be classified by correctly perceiving its color, cut, transparency, luster, dispersion, heft, as well as any phenomenal properties etc. The type of light in which each of the properties is observed is also important.
Beauty, Rarity & Durability: B, R, D, is one of the basic concepts by which a material can be defined as a gemstone.
Beauty: This is a subjective property and is defined by the 4 C’s, Color, Clarity, Cut and Carat size. The presence of a Phenomenon also enhances the beauty of a gemstone.
- Color: In colorless stones, the lesser the color the better the quality. In case of colored stones, the darker and purer the shade of color, the better the quality.
- Clarity: This is defined by the presence or absence of inclusions within a stone which indicates the transparency. This can be more objectively judged by:
- Number of inclusions
- Size of inclusions
- Position of inclusions
- Color of inclusions
- Cut: A stone which has been cut and polished keeping in mind the accurate angles and proportions of cutting, which brings out the full life, fire and brilliance of a gemstone. The beauty of a particular cut can be observed by examining the type of cut, and the symmetry and polish on the crown, pavilion and girdle portions of the stone.
- Carat size: This relates primarily to the weight of a stone. 1 carat = 100 cents/ points = 200mg. The larger is the size with good color and clarity, the higher the value.
- Phenomena: A stone which exhibits a cat’s eye, star or change of color would have an additional beauty and value also.
Rarity: This is an important aspect and may either be due to natural resources or due to man made situations. If a gemstone is available in large quantities, it will not be much in demand. If a gemstone has all the four C’s and a phenomenon, it would be rare. Political and economic conditions in country can control the supply to demand ratio of rough and cut stones. If a stone has a limited geological and geographical occurrence i.e. from a single source e.g. red beryl from Utah (U.S.A.)
Durability: A gemstone must be durable enough to withstand daily wear and tear. This includes the qualities of hardness, cleavage, brittle, toughness, resistance to heat, pressure or chemicals. A softer stone will get scratched more easily. A stone with a number of internal cracks will be less durable. A stone with easy cleavage, e.g. diamond, topaz etc, may split into two if it suffers a hard knock. Perspiration, perfumes, lime juice etc. may damage less resistant gem materials such as pearls.
Color: Perceived by the eye as a result of particular wavelengths of light that the material is capable of transmitting or reflecting. Color is considered by its hue, tone and saturation.
- Hue: Pure spectral colors.
- Tone: That attribute by which a color is perceived as holding a position on a scale from light to dark.
- Saturation: Measurement of the amount of hue in a color.
Color is also considered in terms of a primary and a secondary shade, e.g. bluish green. These factors depend essentially on the type of light used. The same stone may behave differently in different light sources. The light source should be daylight, with a preference for north light (less U.V. radiations). The stone should be examined in reflected white light initially, and then in transmitted light.
Transparency: The quantity and quality of light transmitted by the stone. Use transmitted light to judge.
- Transparent (TP): Transmits light readily, can read through easily, even if colored e.g. Diamond, Ruby, Emerald etc.
- Semi-Transparent (S-TP): Transmits light somewhat less readily, can read through, but slight distortion (somewhat hazy), e.g. Amber and Moonstone.
- Translucent (TL): Transmits light with difficulty, cannot read through, e.g. Chalcedony.
- Semi-Translucent (S-TL): Transmits a very small amount of light, primarily around the edges, e.g. star garnet.
- Opaque (OP): Transmits no light, e.g. Pyrite, Hematite, Lapis Lazuli.
Over colored stones tend to give a misleading idea of transparency. Due to the intensity of color, a transparent stone would appear opaque in reflected light.
Cut: The basic shape or style of fashioning. A few common styles are as follows.
- Facetted: Bounded by flat polished surfaces. Such a stone can be defined by the crown, pavilion and the girdle. The quality of cut can be judged by examining the facets (size, number, shape, symmetry, polish), the cutting angles and the perfection of the girdle shape. A few of the basic styles are:
- Cabochon: A cut stone with a convex or curved form. The flatter portion is considered the base as against the curved surface or dome. Types of cabochons include:
- Single Cabochon – a convex top and flat base.
- Double Cabochon – a convex top and base.
- Other Cabochon styles – Lentil shaped, Hollow cabochon, Reverse cabochon.
- Tumbled: An irregular or baroque form, polished without further shaping.
- Bead: A basically spherical shape, with a hole drilled through the centre. It may be smooth or facetted.
- Cameo: A carved gem with the entire design carved above the girdle edge.
- Intaglio: A carved gem with the entire design carved below the girdle edge.
- Plaque: A flat gem with parallel surfaces. It may have any shape.
Luster: The quality of light reflected off the surface of a stone. Use reflected light to judge.
- Metallic: Metal-like; the highest possible luster. Example: Hematite, Pyrite.
- Adamantine: The highest possible luster for gemstones. Example: Diamond, Zircon, Demantoid Garnet.
- Vitreous: Glass-like; the most common luster for gemstones. Example: Quartz, Beryl, Topaz.
- Greasy: An oily appearance; common for jade. Also seen in some garnets, peridot etc.
- Resinous: Similar to greasy; used to describe the luster of amber.
- Waxy: Little reflection; similar to the luster of a candle or a fingernail. Example: Turquoise, Chalcedony.
- Dull: Very irregular, fine-grained surface common on unpolished, opaque stones. Example: rough Turquoise, Malachite.
- Silky: Reflection off fibrous structure (sheen). Example Tiger’s eye.
- Pearly: Due to structure (sheen); common to pearl, rough moonstone, cleavage surfaces etc.
Dispersion: The breaking of white light into its component spectral colors. Observed as flashes of spectral colors, known as fire. A few stones have eye-visible dispersion which can be a useful identifying characteristic. The presence or absence of dispersion in diamond simulants is important in their identification (example Strontium Titanate – very high dispersion; Y.A.G. – barely visible dispersion).
Phenomena: These are optical effects which occur in some varieties of certain species.
- Sheen: The moving white light optical effect observed below the surface of the stone as a result of reflections from layers within. Example: Moonstone
- Opalescence: The milky sheen observed in opals.
- Chatoyancy: A cat’s eye effect which is caused by reflections of light from parallel needle-like inclusions. This is observed as a bright line moving over the surface of a polished cabochon at right angles to the direction of the parallel inclusions. Example: Chrysoberyl, Apatite, Quartz, Enstatite etc.
- Asterism: A star effect which is caused by reflections of light from two or three sets of parallel needle-like inclusions. This is observed as a four rayed or six rayed star over the surface of a polished cabochon. Example: Ruby, Sapphire, Garnet, Spinel, Diopside, Quartz etc.
- Aventurescence: A glittery effect which is caused by reflection of light from inclusions of tiny platelets or flakes of another mineral. Example: Aventurine Quartz, Goldstone, Glass and Sunstone feldspar.
- Change of Color: A noticeable change in the body color of a stone when it is observed under daylight and incandescent light. This is caused by selective absorption and selective transmission of light. Example: Alexandrite, Sapphire, Garnet etc.
- Play of Color: The spectral colors produced by a combination of interference and diffraction of light. Example: Opal, Labradorite etc.
- Iridescence: In oil on water effect, giving rise to prismatic colors caused by interference of light from thin films of gas or liquid, Example: Iris-Quartz.
- Orient: The prismatic colors caused by interference and diffraction of light in the scale-like structure of pearls and shell.
- Adularescence: A billowy blue or white light in feldspars caused by scattering of light along with an interference effect. Example: Blue sheen in Moonstone.
How to identify fracture & cleavage?
- Use oblique, reflected light.
- Observe the surface of the stone, especially in areas prone to damage (e.g. the girdle and culet).
- Note any external breaks.
- Note any internal, parallel, iridescent smooth cleavage fractures.
Fracture – any irregular break; identify according to type and luster (the luster of fractures can be a key identifying characteristic).
Types of Fractures:
- Conchoidal: Shell-like; a smooth, curved break often with concentric ridges; the most common fracture for transparent crystal materials and glass.
- Splintery – Fibrous appearance due to structure; similar to a break on wood with the grain.
- Granular – Sugary; a grainy or coarse fracture is common to crystalline aggregates.
- Even – Relatively smooth, flat break. Resembles cleavage, but with no step – like breaks.
- Uneven – A rough, irregular fracture that does not fit into any of the above categories.
Cleavage – A smooth, flat break; attempt to find the number of directions; use as proof if this property is correctly identified in the form of smooth internal parallel cleavage fractures or as an external feature.
Parting – False cleavage; a smooth flat break normally following twinning planes; visually resembles cleavage.
Heft: Each gemstone has a characteristic specific gravity which is denoted by a number.
Knowledge of this property enables a jeweler / gemologist to approximate the specific gravity with respect to the weight of the stone. By judging the loose stone in the hand, one can determine whether the stone is light, medium or heavy in weight. With practice, one can easily separate stones such as
- aquamarine (lower heft) from blue topaz (heavier)
- rock crystal quartz (lower heft) from white topaz (heavier)
- diamond (lower heft) from synthetic cubic zirconia (heavier)
Further one can estimate the size of a stone by relating to the heft of the stone. Hence a two carat aquamarine will be bigger in size than a two carat blue topaz.
Some common terms:
The International Confederation of Jewellery, Silverware, Diamonds, Pearls and Gemstones (CIBJO) defines synthetics, imitations and composite gemstones as follows:
- Synthetic Stones are crystallized or re-crystallized products whose manufacture, by which ever method, has been caused completely or partially by man. Their physical and chemical properties and/or their crystal structure essentially correspond to their naturally occurring counterparts.
- Imitation Stones are simulations of natural or synthetic stones, products made entirely or partially by man. These stones imitate the effect, color and appearance of natural gemstones or synthetic stones without possessing their chemical composition and/or their properties and/or their crystal structure.
- Composite Stones are crystalline or amorphous bodies composed of two or more parts assembled not by nature, but by cementing of the pieces or by other artificial methods. Their components may be either natural gemstones or other minerals, or synthetic stones or a chemical product.
- Reconstructed Stones are artificial products, manufactured by melting, bonding, or fusing materials to form a coherent whole. Natural and synthetic chips or waste materials are commonly bonded, as is seen in turquoise, coral etc.
A Simulant may be a…….
Natural; Synthetic; Imitation; Composite; Reconstructed; Surface modified
Leave a Reply
You must be logged in to post a comment.