GIL

The term ’Natural Gem’ refers to a mineral possessing Beauty, Rarity and Durability; having some specific chemical, optical and physical properties and has grown naturally within the earth crust. Examples: Ruby, Sapphire, Emerald, Diamond, Garnet, etc.

‘Synthetic Gem’ refers to man-made material with similar chemical, optical and physical properties to that of corresponding natural gem. Example: Synthetic Ruby having all the properties similar to the natural counterpart.

‘Imitation’ refers to a look-alike material to that of a gem it is being imitated, without possessing its chemical, optical and physical properties. It may be a natural or a man-made material. Example: Red Spinel whether natural or synthetic can be considered as an imitation for a Natural Ruby, same way a glass or any other material with similar appearance can be an imitation for ruby.

The beauty or quality of a stone is judged by the 4C’s, i.e. Colour, Clarity, Cut and Carat Weight. Colour is judged by Hue (pure spectral colour), Tone (lightness or darkness) and Saturation (intensity), under proper lighting conditions. Clarity is judged by the presence or absence of inclusions considering their size, position, colour and number. Evaluation of cut involves the shape, cutting style, proportions, symmetry and finish which affect the overall brilliance. Carat weight refers to the size of the stone, larger the size higher the price.

For coloured stones like ruby look for the purer and richer red colour with minimum of secondary purple or brown colours while in case of diamonds (especially colourless) look for the purest white without any yellow and brown tints. The colour of a stone is also affected by the lighting conditions; for e.g. a ruby appears richer under an incandescent light as compared to daylight fluorescent lamp and on the other hand a blue sapphire appears better under daylight fluorescent lamp.

Gem materials with lesser number of inclusions in general fetch a better price. Increase in the visibility of inclusions, price decreases; the affect of inclusions is more reflected in a colourless stone like diamond as compared to coloured gems. However, few stones like sagenetic (rutilated/ tourmalinated) quartz, demantoid garnet or amber are known for their classic inclusions and hence the presence of inclusions increases beauty of these stones.

In addition to the 4 c’s, optical effects or phenomenas also add to the beauty and hence additional price to the gems as in case of cat’s eye, star, aventurescence, play of colour, colour change, etc.

A synthetic is a man-made product with optical, physical and chemical properties similar to its natural counterpart, while a glass (natural or man-made) has its own properties and only has the ability to imitate any natural gem not its properties.

A greenish yellow stone purchased by a trader as a ‘lemon topaz’ and is certified as ‘citrine’ because of the fact that the purchased stone is a citrine but wrongly called as topaz. Both are two distinct species; Quartz is pure Silica, while Topaz is Hydroxyl Fluorosilicate of Aluminium. Both the materials can be easily differentiated on the basis of Heft, Quartz is much lighter as compared to Topaz; SG of quartz is 2.65, while that of Topaz is 3.54. Topaz has a better life because of higher RI of 1.620- 1.630, while quartz is comparatively duller with RI of 1.540 – 1.550.

When viewing the stones under a 10x or a higher magnification, Quartz commonly exhibits brown or yellow colour bands in rhombohedron orientation, 2 or multiphase inclusions, liquid fingerprints, needles, crystals, etc. Topaz exhibits cleavage planes (a flat break inside or on the surface) one of the most important and conclusive differentiating feature, non-miscible liquid inclusions, iron stained cracks, etc.

The term “lemon topaz” should not be used for “citrine or lemon coloured quartz” as it is just a misnomer.

A basic set of equipment a trader requires to get rid of day-to-day problems or for general packet lot separations are:

• Polariscope (with conoscope)
• Dichroscope
• Specific Gravity Liquids (Bromoform and Methylene Iodide)
• 10X lens or any other magnifying tool – also a microscope- if affordable
• Chelsea Filter (optional)

Using these instruments with proper care / application one can identify almost any stone. But some other features should also be observed like, life, fire, abrasions, type of fractures/ cleavages, etc.

Alexandrite is known for its characteristic colour-changing effect from green in daylight to red in incandescent light. There are some factors / criteria to be kept in mind while judging the colour change effect:

• The most important factor is that the stone is observed in reflected light and not in transmitted light.
• Always keep the light source constant such as the daylight non-fluorescent tube and an incandescent lamp.
• The distance of the stone from the light sources, whether daylight or incandescent must be constant.
• The stone to be examined must be kept on a white background with table facet towards the light sources and observed with light reflected off it.
• If and only if there is a perceptible change of colour from one source to another, the stone can be considered as a colour changing.
• The percentage of change however can only be an approximate idea of whether it is 50% or 75% or 100% unless predetermined with a graded master set.
• Take a note of iron-stained fractures, which enhances the effect.

Ruby and Sapphire both belong to the same species- Corundum; the only difference between the two is the colour shade and is much subjective. Red coloured corundum is considered as ruby, and all other colours are sapphires, which includes pink. This problem of calling a stone as a ruby or a sapphire is encountered in case borderline shades like purple pink or purple red where the purple tone is quite strong and the shade shifts from pink to red in various types of lighting environment.

To judge the difference between the two, stone should be viewed under a standard daylight against a dull white background and the stone can be considered as:

Ruby if following observed:

• Pure red.
• Purple red.
• Purple red with a weak bluish shade inherent in the stone.
• Purple red with a weak brownish red inherent in the stone.

Sapphire if following observed:

• Purple red with strong pink shade inherent in stone.
• Purple red with distinct blue shade as visible on the table surface.
• Purple pink
• Rose pink with nil or very weak red shade inherent in stone.
• Purple red with a peach shade inherent in stone.

Some stones may fall on the borderline shade between ruby and sapphires, in those cases, the stones may be considered as a ruby or a sapphire by different labs if consistency in the lightening conditions is not maintained.

Emerald and Green Beryl- both belong to the same species- Beryl. The difference between the two is the presence or absence of Chromium, which is detected by a spectroscope. If a stone has chromium content, spectroscope will exhibit lines in red region that concludes the stone as an emerald while in case of green beryl there is no absorption in red because of the lack of chromium content. In a single rough crystal, it is possible that one end has got the chromium concentration and another one none.