Gemstone Appraisal Reference & Tutorial

Many types of instruments are used in the laboratory for examining gemstones and other valuables. The terms expressed in the documents created at the laboratory may be perplexing to clients. We've created a tutorial to explain the equipment used at Guild Laboratories and some terms which appear often in the documents we create.

Appraisal An evaluation of jewelry, gemstones or other valuables, expressed in dollars (or other monetary values) to be used for insurance, estate or other purposes. The appraisal is a researched estimate of retail replacement value, or for another appropriate market. A properly prepared appraisal conducted by a qualified appraiser affords protection in the event of loss or damage and is critical for settling probate, estate and other types of property distributions.

Binocular Microscope A critical piece of magnifying equipment in a professional laboratory.  The gemological microscope has twin objectives & oculars which provides three dimensional viewing, various magnification lens, plus variable illumination sources to examine the exterior and interior of an object.

Branded Gemstone or Diamond A proprietary, often patented gemstone cut making the stone recognizable as a unique shape. Branded cut stones may also bear identifying laser inscriptions on the girdle (circumference).

Carat Weight A measure of weight, rather than the size of a gemstone or diamond as expressed by carats.

Certificate A written report describing the results of examination of a gemstone, diamond or other valuables detailing the weight, proportions, color and clarity grades, and any other variables pertaining to its identity and quality.

Chelsea Filter A small handheld device assists in discriminating between natural green emerald and other green gems, glass or gem simulants.

Clarity Grade A method of expressing relative levels of clarity in a diamond. Flawless (FL) to Included 3rd level (I3), or lowest.
          DIA Clarity Grading: FL-IF-VVS1-VVS2-VS1-VS2-SI1-SI2-I1-I2-I3.

Color Grading Scale (DIA) Colorless diamonds are graded on a scale beginning with the letter D and continue to the end of the alphabet.

D-E-F Colorless
G-H-I-J Near Colorless
K-L-M Faint Yellow
N O-P-Q R Very Light Yellow
S-T-U -V-W-X-Y-Z Light Yellow

Diamond A unique gem-mineral composed essentially of only one element: carbon. It is the hardest of all known natural substances ranking10 on the Mohs scale. It occurs in colorless form, called "white" diamonds, but also occurs naturally in a wide range of colors.


Dichroscope A small and very portable laboratory instrument also used in the field to test for pleochroism-- a trait observed by a gem changing colors when viewed through its lens while being turned. This is one test for gem identification.

Dispersion.  The term used to express a transparent gem or diamond's ability to separate white light into the spectral colors; also referred to as fire.

Facet A polished surface placed on a diamond or gem.

Face-up Color  Apparent color observed in a diamond or gem viewed in a direction perpendicular to the table or how it would appear in its setting.

Fancy Cut Any gemstone shape or cutting style other than a Round Brilliant cut. This includes pear, heart, emerald cut, marquise, princess, trillion, cushion etc.

Fancy Color A diamond with an attractive natural body color other than very light yellow. This includes pink, blue, brown, fancy yellow (canary) and other rare colors. Any color sapphire other than blue is called Fancy Color sapphire.

Fire See Dispersion.

Flaw A catch-all term referring to any internal or external characteristics on a cut diamond or gemstone, also called imperfections or inclusions.

Flawless (Fl) A diamond or gem with no internal or external flaws or blemishes when viewed under 10X magnification.

Fluorescence The emission of light from a diamond under a source of controlled ultraviolet lighting. This is part of an identifying test performed on some diamonds.

Four Cs An easy to remember quality assessment for diamonds: Carat weight, Color, Clarity and Cut.

Fracture-Filled Stone An enhancement technique using materials which fill surface-reaching fractures. This process is not considered permanent, and a full disclosure of these treatments by the seller must be made to the consumer.

Full Cut (or Brilliant Cut) A round cut diamond with the total of 58 facets.

Gem A cut and polished stone that possesses the necessary rarity, beauty, utility and durability for use in jewelry or for a collection.

Gem Identification A series of systematic tests performed in a laboratory or other setting as allowed until an absolute separation (identification) is made of the stone in question.

Gemology The science and study of all issues relating to gemstones including their identification, description, origin, and grading.

Gram The weight in grams of metal used in jewelry or decorative items. 28.35 grams = 1 ounce Avoirdupois

Hardness A relative ranking of a mineral's resistance to being scratched. The Mohs scale developed in the 19th century is the most accepted method for comparison of relative hardness. Diamond, the hardest known substance is rated 10 on the Mohs scale, and talc is the least hard mineral, ranking 1 on the Mohs scale.

Imperfection Any internal or external flaw or blemish on a polished diamond or gemstone, also called an inclusion.

Irradiated Diamond A diamond that has been treated with safe irradiation and annealing to increase its attractiveness and sale-ability.

Karat A measure of gold purity or fineness, usually stamped on a piece of jewelry with the mark K or KT after a number. It represents the relationship of pure gold to the total metal alloy. 24K means pure gold. 14K equals 58.5% pure gold in the total metal.

Leveridge gauge A millimeter dial micrometer that measures mounted and loose diamonds and gemstones of any shape and size.

Loupe A handheld 10X optical device, corrected for spherical & chromatic aberration used for examining gemstones.

Microscope See Binocular Microscope

Millimeter (mm) Metric system unit of 1mm = 0.001 meter. 6 mm measures nearly 1/4 inch.

Millimeter Gauge A precision tool used for accurately measuring (to 0.01mm) the overall dimensions of gemstones or of jewelry in millimeters.

Mohs Scale See Hardness

Old European Cut Describes the earliest form of circular shaped full brilliant cut diamonds. It is distinguished by a small table, a high crown and great overall depth contrasted to modern round cuts.

Old Mine Cut A term for an early form of brilliant cut diamonds featuring a square-like or cushion-shaped outline.

Polariscope An optical instrument used to identify a gemstone as having single or double light refraction; one of several tests used in gemstone identification.

Refraction This is how a gemstone handles light rays entering it, and is expressed in a stones' RI or Refractive Index.

Refractive Index (R.I.) A measurement of how light rays change direction (related to speed) as they enter or exit a gemstone.  There is a direct correlation between the higher RI's and a stones' brilliance.

Refractometer An optical instrument used in the laboratory for measuring the refractive index of a gemstone.

Rough Diamond A diamond crystal in its natural state as found in the earth.

Slightly Included (SI) A clarity grade in a diamond, which shows more inclusions than VVS or VS but less than Included. Diamonds of this grade are usually very attractive to the naked (unaided) eye.

Specific Gravity A way to measure and articulate the relative density of gemstones to that of water.

Spectroscope This optical instrument reveals the unique absorption spectrum of gemstones, and is used for identification and separation of natural and synthetic gems etc.

Treated (Enhanced) Diamond A natural diamond that has been coated, filled, laser drilled or otherwise treated to improve, change its color, or its appearance.

Ultraviolet Light Electromagnetic wave lengths shorter than that of visible light. The use of ultraviolet light is important in the study of diamonds and colored gems because of the (identifying) fluorescent effects seen in some gems.

Very Slightly Included (VS) and Very Very Slightly Included (VVS) These grades identify imperfections in a diamond which are few, small and very difficult to see even with the aided eye at 10X magnification.

X-Ray Fluorescence Some diamonds fluoresce from exposure to X-rays; usually in varying intensities of blue.

Zirconia cubic (CZ) A created material without a natural counterpart that is often used as an inexpensive diamond simulant.

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