TOOLS FOR DIAMOND GRADING

The following tools are used in diamond grading: balance, tweezers, loupe, source of standard white light (lamp), ultraviolet lamp, gauge, color materstones, microscope, standard white paper, and cleaning liquid and materials.

Before examination of a stone, its surface should be cleaned. To accomplish this, a diamond is washed with alcohol, wiped with a special cloth, or boiled in acid. Diamonds are stored in paper packets, better if the packets are specially designed for this purpose.

The following gemological tools may also be helpful for diamond identification: Sorting diamonds is carried out with a special shovel and a sorting tray. refractometer, polariscope, conoscope, spectroscope, and immersion microscope. Sorting diamonds is carried out with a special shovel and a sorting tray.

Shovel
 
Sorting tray
 
Tweezer

pincet.jpg (38598 bytes)There are various kinds of tweezers. Black tweezers are recommended for grading in order to avoid reflections inside a stone. The tweezers should be soft enough. Tweezers differ in length, thickness of ends (lips), and hatching on them. Tweezers with clips are also used. Figure shows how a stone can be held with tweezers. A stone should be held close to the tips of tweezers, otherwise reflections of the tweezers may appear inside the stone. The tweezers should be held with a minimal effort, just sufficient for fixing a stone. Overpressing on tweezers' lips can result in "shooting" a stone from the tweezers. The tweezers should be held without tension. A stone can be tilted or rotated together with the tweezers for successive examination of all its zones from different sides.


Loupe

Gemologists use triplet-type, 10 X loupes, without distortion of colors and image throughout the vision field (achromatic-aplanatic lens), 18-20.5 mm in diameter. Loupes with black frames are preferred. Loupes with tenfold magnification and a measuring scale are used for the estimation of linear dimensions of a stone. When a diamond is examined with a loupe, it is important to provide a proper position of a stone held with tweezers relative the light source and the "loupe eye". The light source is set opposite the eye above the table approximately at or slightly below the eye level and is orientated so that the direct light falls on the stone but does not strike the eye. The loupe is fixed by the right hand at the right eye, with its frame touching the eyebrow. A stone is held with tweezers at a distance of about 5 cm in front of the loupe, so that the specimen is in focus and in the center of the field of vision. Both hands should be in a contact with each other to avoid trembling of the loupe or stone. The elbows may be placed on the table.


Lamp

Standard white light source without predominance of the blue or yellow tint is required for diamond grading. The lamps manufactured by Dazor or Eickhorst ("Dialite UV", "Colorscope"), and others with a standard color temperature 5500-6500K (D55-D65)are commonly used. Some lamps contain inner ultraviolet sources. A diamond is held directly beneath the lamp near the light-shade boundary.


Standard white background

Standard white paper sheets of various size, sometimes folded to the U-shape, are used for precision color grading of diamonds. Gray background without blue or yellow tint is tolerable.


Ultraviolet lamp

Ultraviolet lamp enables the observation of fluorescence ((luminescence) of diamonds in UV-rays (wave length 365 nm). A diamond is exposed to ultraviolet irradiation in a darkened room, and the luminescence color and intensity are determined visually. In some cases, luminescence intensity masterstones are used.


Leveridge gauge

laveridge.gif (62510 bytes) This device designed for the measurement of linear dimensions of gemstones, including mounted stones, is often called "Leveridge" for the name of its inventor. It may also be called gauge or estimator. This device has a measuring limit of 23.5 mm and is accurate to one tenth or one hundredth of a millimeter. Electronic and mechanic Leveridge gauges are produced. When the Leveridge gauge is used, sharp clicks on a stone must be avoided, since they can result in chipping off the culet. For more see "Leverige usage".


Microscope

A gemological binocular microscope , is commonly used for diamond grading. It provides three-dimensional observation, changing magnification, and two lighting options:

  1. reflected light (for examination of stone surface) and
  2. dark field illumination (for revealing internal defects). Some modifications are equipped with a fiber optic, which enables local illumination.

When a stone is examined under the microscope, it should be held with tweezers or with a special holder. The holders can be spring-type and vacuum-type. Depending on the microscope modification, magnification can be varied continuously (ZOOM system) or in steps. The maximum magnification factor used in such microscopes ranges up to 40-50. A HRD gemological microscope has a special graduated net on its ocular to determine the size of inclusions.

Some gemological microscopes are supplemented with an additional lighter with fiber optic, which enables extra illumination of some stone details.


Balance

vesi.jpg (52407 bytes)A special balance is used for gemstone grading (1st or 2nd precision category). The weight of diamonds is measured in carats (ct) with an accuracy of one hundredth of a carat (the third decimal digit is rejected; if it is 9, the value is rounded up). Balances manufactured by Mettler of Satorius are commonly used. A portable electronic balance with a carat scale or even an ordinary pharmaceutical balance can be used for approximate weight estimation. The weight of a mounted diamond is calculated through weight-estimation formulas, if the stone cannot be unmounted.


Proportionscope

This device is used to determine the diamond proportions and evaluate the cut parameters of the stone by comparing its projection to the schematic image of the ideal diamond. Proportionscope enables the measurement of crown and pavilion angles, table size, crown height, pavilion depth, girdle thickness, and the deviation of the diamond shape from round.


Set of color masterstones.

A set of masterstones is recommended for determination of diamond color. The masterstones are previously calibrated against initial masterstones and their colors are measured with a photometer. Diamond masterstones are preferable, although CZ masterstones may also be used. The masterstones in a set should be similar in size; those with a diameter of 6.5 mm (as 1-ct diamond) or 5.2 mm (as 0.5-ct diamond) are most commonly used. Grading of colored diamonds requires special masterstones. If these are not available, color charts (Munsell color tables) are used.


Diamond-detector

This device makes it possible to differentiate diamonds from their imitations by measuring the heat conductivity of a specimen. Some devices measure reflective ability or both parameters (duotesters). There are digital and indicator detectors; the latter type is more preferable. The detector enables an expert to set diamond apart from all its imitations, except moissanite. Before examination of a stone, its surface should be cleaned. To accomplish this, a diamond is washed with alcohol, wiped with a special cloth, or boiled in acid. Diamonds are stored in paper packets, better if the packets are specially designed for this purpose. The following gemological tools may also be helpful for diamond identification: refractometer, polariscope, conoscope, spectroscope, and immersion microscope. Sorting diamonds is carried out with a special shovel and a sorting tray.