- Gemprint International
- Polariscope - single/double refraction
- Specific weight
- Observations with the loupe
- Brilliance and dispersion
- Internal Features
- Diamond girdle
Measuring of refractive index with the help of a Refractometer and Reflectometer
All mentioned substances can be measured on the Refractometer, as long as their refractive index is below 1.81. The limitation to this value is the dependence on the refractive index of the contact fluid which is used between the stone to be tested and the glass prism of the Refractometer. Substances with a refractive index above 1.81 show a negative reading, i.e. nothing shows on the Refractometer scale.
A new instrument which measures the refractive index of highly refractive substances is the Reflectometer which is based on the measuring of the reflection intensity of a - if possible not too small - facet of a stone. The, instrument can be recommended for a quick decision as to: diamond or imitation; it should be routine when accepting stones or jewelry for repair.
The stone to be tested is laid table down onto the measuring opening of the Reflectometer. In the case of set stones a solid deposition of a facet or the table is important. Pressure on a key releases a ray of light, which according . to the laws. of. reflection (see page 153) is reflected from the surface of the stone and enables the intensity to be measured; this is characteristic of every stone, depending on its refractive index.
The accuracy of the measurement is dependent on the quality of the surface and the polish of the stone. Therefore the stone should be well cleaned and free of surface marks.
The dial of the measuring instrument shows a zone which is valid for a special gem variety, however, some over-lapping can occur. The result is not read off a scale with refractive index number, but off a scale showing possible variety: of gemstone. As results can overlap, this testing method does not always give definite information about the type of stone. But it will always inform as to whether the stone to be tested is a diamond or an imitation.
As diamonds are good conductors of heat, they feel cold to the touch as opposed to other gemstones, imitations and artificial products. The thermal resistance at a diamond is around one hundred times less than that of most other gemstones and imitations.
The heat conductivity of a substance can be expressed by a number which specifies the amount of heat which penetrates within a certain time unit the area unit of the substance of a thickness of 1 cm, when the difference of temperature between the two surfaces amounts to 1° Kelvin.
Thermolyser / 2
The Thermolyser/2 (Eickhorst system) is a very handy and compact instrument for distinguishing diamonds from imitations. It can be easily brought along On purchasing trips. It works from the mains and on batteries (Fig 401).
|Fig 401 Thermolyser/2|
Like other instruments the Thermolyser/2 is based on the measurement of thermal conductivity.
The instrument consists of a probe, energy source, amplifier and digital display. The probe is the heart of the instrument. A heating element delivers a constant temperature to the copper-constant-probe. When the gold-plated tip of the probe is placed on the gemstone, the latter takes away some of the heat from the probe, This difference in temperatures is processed by the electronic system.
The direct LED readout scale of the Thermolyser/2 has a high light-transmitting capacity and is quite visible even under bad lighting conditions. It shows diamonds in the green range and imitations in a red field.
During contact with the surface of the stone to be examined, the read-out "runs" from the left side of the scale to the corresponding position in the red or green area, stays there for a short while, and then runs back again slowly to the initial position on the left edge; A longer contact with the stone to be examined therefore does not lead to an unambiguous result. In case of doubt it is advisable to repeat the measurement.
Should the test tip make contact with the setting of the stone to be examined a buzzing sound operates as a warning. In this case, the read-out value on the scale does not give the correct information, and the measurement has to be repeated.
WARNING: One requirement is that one finger should touch the contact rail on the upper side of the instrument, while another finger should hold the gemstone or setting. It is a good idea to tap the setting briefly before the beginning of a measurement in order to check the warning signal.
Reproducible results are the most important aspect of a measuring instrument. Uneven parts of the surface which are scarcely visible, or a test tip which is rounded off or laid on obliquely, can be a source of error. Good results can be obtained, however, with practice and the exercise of care. But measurement of a gemstone should be carried out several times in order to obtain an average value.
It has proved useful to test the operation of the instrument occasionally with the aid of well known gemstones. Here it is extremely important to use comparison stones of comparable size.
The battery-operated Presidium diamond tester is also based on measurement of thermal conductivity. Set and loose stones can be identified - from 0.02 ct upwards. Measurement accuracy can be checked at any time with the aid of the built-in test disc.
This very reliable, instrument can be used for assuring the identity of a stone Or for determining its genuineness. A fine laser beam is directed at the table of the brilliant (set or unset) to be examined, from which light reflections, which are quite individual in character for each stone, are projected onto a small screen. However, the opportunities for testing are limited to brilliant cuts.
The identity images, which are similar to fingerprints, are a very useful addition in an expert opinion. They also enable a "passport picture" to be produced in the presence of a customer, as, for example, when a diamond gemstone is being accepted for repair. This picture can be reproduced at any time from the same stone. Although the instrument is very easy to operate, it makes sense to use it only where daily practice creates a permanent need for it.
The instrument consists of a horizontal 1 m x 0.50 m base, a laser beam, an optical lens system, and a Polaroid film cassette with small screen connected in series.
Tile brilliant is irradiated with a fine laser beam in the table-culet axis and the resulting light reflections are recorded on Polaroid film. The position and arrangement of the light spots depend on the proportion and symmetry of the stone. The number of reflections is directly connected with the refraction of the stone, so that a distinction can be drawn at the same time between a diamond and an imitation (Figs. 404 and 405).
|Fig 404 Brilliant cut diamond||Fig 405 Brilliant cut Zirconia|
With unset stones care should be taken that the table is approximately parallel to the longitudinal section of the lens.
The stone is placed in a small black clamp in front of the laser beam.
The diamond then reflects an image which is composed of numerous small light spots without any apparent symmetry. One dot however stands out: it is the culet of the diamond. Only this spot moves horizontally or vertically. All the other spots move in circular fashion. This scintillating spot (culet) has to matched up with hole in the middle of the screen. Only then can a photograph be taken.
The reproducibility of such a photograph is guaranteed, as the angle to the table-culet axis, at which he diamond is brought in front of the laser beam, is not important. It can be taken as certain that each picture taken during rotation around the axis will have the same image.
Detection of single and double refraction, rasp. anomalous double refraction with the help of a Polariscope
Under polarized light singly refractive substances do not change in brightness when the stone is rotated around 360°; a doubly refractive stone will become lighter four times when rotated around a full circle. All anomalous doubly refractive substances show a spotty or flame-like brightening.
With the help of hydrostatic balances, the specific gravity can be calculated from the measured loss of weight which a stone is subjected to when completely immersed in liquid. The formula is as follows:
Specific weight = Absolute weight / Loss of weight
Specific weight can also be ascertained with heavy liquids of known specific gravity. The stone is immersed in the liquid:
- if the stone sinks, it is heavier than the liquid
- if the stone rises, it is lighter than the liquid
- if the stone is suspended, its specific gravity is the, same as that of the liquid.
The simplest procedure is to use a solution with the specific gravity of diamond = 3.52; then only a diamond will remain suspended, all other imitation stones will either sink or rise. colorless topaz with a specific gravity of 3.53 ± 0.04 can be an exception. But in contrast to the singly refractive diamond, topaz is doubly refractive.
Because of the standard form of cut of a brilliant, there is a constant ratio between its weight and its size; therefore one can estimate the weight of a brilliant from its girdle diameter. Substance whose specific gravity differs considerably from that of a diamond do not confirm to this ratio. Thus, the artificial product "Zirconia" weighing one carat. is much smaller than a diamond of the same weight, because the specific gravity of Zirconia is 6.00 compared to 3.52 of diamond.
Because of the optical characteristics of a diamond, an expert cans judge whether a diamond is genuine or not by looking at it with a loupe or under a microscope.
All mentioned substances are much softer than diamond, and show often scratches on the surface, damaged facet edges or damaged culets. Soft substances with a hardness below 6, often do not have such sharp facet edges as a diamond, but have them slightly rounded.
With n = 2.417 a diamond has a very high refractive index and is singly refracting. The total reflectance resulting from the high refractive index is also very high.
Only with synthetic rutile is refraction (2.62 to 2.90) even higher, and, in contrast with a diamond, rutile is highly doubly refractive.
If, for example, a diamond brilliant is examined against a light source, it appears to be opaque, i.e., it appears black, because of its high total reflectance. On the other hand stones with a lower refractive index and proportioned like a diamond brilliant appear translucent. The lower the refractive index, the more transparent is the stone.
This can be easily demonstrated with two different stones which are laid with their tables over print or a line. In the case of a diamond brilliant the print cannot be looked through because of high total reflectance (reflection), while in the case of a stone with a low refractive index, the print or line shows through in every facet of the pavilion around the tip.
As brilliance depends, among other factors, on the refraction and hardness of a stone, stones with a low degree of hardness and a low refractive index do not achieve the brilliance, "hard" luster, or transparency of a diamond. Synthetic Rutile and Fabulite, both artificial products, show strikingly high dispersion as compared with a diamond. They have a "indistinct" oily appearance which is characteristic for these products.
Certain internal features can be so typical in diagnosis that they provide information regarding the variety of a gemstone.
Twinning lines and growth lines, for example, are quite typical of a diamond, as are certain structural features due to irregularities in the lattice structure of the carbon atom which is otherwise highly organized structurally.
Moreover certain kinds of cracks, for example, cleavage and fracture cracks are typical of a diamond to an extraordinary degree.
Other mineral inclusions such as olivine, garnet, pyroxene, spinel, ruby, rutile and zircon are known as guest minerals in diamonds.
The nature of the girdle is so characteristic of a diamond that by examining it closely a specialist can determine whether he is dealing with a diamond or an imitation.
A bruted girdle in a diamond brilliant is fine-grained with a waxy t6 silky dull bright texture or it is polished or facetted (see Fig 308), i.e., has any number of small facets joined together in a row.
Also characteristic of a diamond is a porous girdle with small "fringes" which originate from excessive rubbing or rounding during the cutting of the stone. This can give rise even to small fractures which, with their step-shaped appearance due to the cleavage of a diamond, are also typical.
A very clear sign of the genuineness of a diamond are the residues of the natural crystals, the so-called "naturals", which always indicate the surface structure of a diamond crystal in the shape of either a fine line or small triangles and squares.
Doublets can be recognised by the separation line between crown and pavilion along the girdle, or one can observe tiny air bubbles in the separation plane. Also differences in luster point to the use of two different substances.
|Diamond imitations||Tabular summary of diamond imitations with distinguishing features|