Dyeing (Colored Impregnation)
For simplification, enhancement of a gemstone using any coloured material in the filling of fractures, cavities or voids and for the compaction of porous gemstones, is considered under a single heading. This category also includes techniques which involve a chemical reaction within the stone which may or may not be formed as a result of heating.
Nature of gemstones:
- Any stone which contains surface reaching fractures.
- Porous stones.
- Materials used: Oil (cedar wood oil, linseed oil) Canada balsam, wax, plastic, polymer, resin, glass etc. along with additional colour, is introduced as filler.
- Possible effects: An overall enhancement of colour which is localized at fractures in fracture filled stones and in pores in porous stones. The process can involve an additional treatment such as heating and chemical reactions to stabilize the coloured impregnation.
Stones enhanced: Almost any stone can be enhanced in this manner. For convenience, only a few of the most routine stones are considered with respect to the materials used.
- Coloured oil / resin/ wax / polymers: The traditional technique of using natural vegetable oils with colour known as Jhoban, as fracture filler for emeralds, rubies, quartz etc. is a major issue in the international markets. This has assumed importance due to the fact that the materials are being used as fracture fillers these days, also includes chemical dyes, paints, inks etc. which in most cases are not the natural vegetable colours. Coloured wax and polymers are commonly used to improve the colour of coral, jade, turquoise, etc.
- Colour as a result of chemical reactions & / or heating: This category includes stones said to be dyed. A dyed stone is one in which the solvent evaporates and leaves the dye or pigment within pores, cracks or gets attached chemically to the dyed material (Ref: Gem Enhancement by K. Nassau) The most common examples include dyed chalcedony (in almost all colours) dyed quartzite, dyed corundum, sugar / smoke treated opals etc.
Identification: The criteria used to identify coloured impregnations are very similar to that for colourless impregnation.
- First locate the surface break which appears as a thin line on the surface.
- Locate the direction in which the fracture is oriented within the stone.
- Reflect light off the fracture surface and observe any or all of the following.
- Trapped gas bubbles and lumpy fillings in the fracture due to incorrect filling.
- Colour concentrations in fractures.
- Magnification: Under low magnification the impregnated porous stones may show a speckled or slightly sugary appearance along with colour concentrations.
- U.V. Fluorescence: Some filler may fluoresce distinctly different from the body colour of the stone.
- Hot Point: This may help to identify different impregnating material - polymer, wax and plastic will give a peculiar and distinct odour and may melt where the hot point is applied.
- Other techniques: To conclusively detect the exact nature of the impregnated coloured materials it is necessary to resort to Infra-Red and / or, Raman Spectroscopy, X-ray Diffraction or SEM (Scanning Electron Microscope).
- Fracture filled:
Some specific examples of gemstones:
- Turquoise: Often porous and is sealed with wax or plastic to stabilize and improve the durability and colour. A new treatment is that known as the Zachary treatment.
- Coral: Often porous and is impregnated with wax or resin to stabilize and improve the durability and colour.
- Black Onyx: Mostly agate which has been impregnated with sugar, which is then carbonized by acid treatment. Similarly green onyx, blue onyx etc. are mostly agates which have been dyed using specific chemicals.
- Lapis lazuli: Very commonly dyed to obtain a uniform deep blue colour, maybe coated with a fine layer of wax, maybe heated to stabilize the chemical reaction.
- Jade: A specific example is that of B-Jade. This is low quality jadeite which is initially bleached and then polymer treated to produce a more uniformly coloured and durable material. Jadeite is dyed in many shades, green and lavender being the most common.
- Opal: Two well know techniques - sugar and smoke treatment improve the phenomenon of play of colour by giving a darker background to the opal. Opals are polymer impregnated to enhance the colour and also the durability of the stone. Sugar treatment is identified by the presence of black peppery effect just below the surface of the stone, while smoke treatment is identified by the presence of cloudy effect.
- Corundum - ruby / sapphire: Glassy fillings in ruby are of two types. Smooth fillings in fractures, twin planes and cavities which exhibit a high mirror like reflection and a difference in the lustre at the surface break. During the heating process, ruby is coated with borax powder. This powder goes into a melt form and enters into any surface reaching fractures. This has the appearance of flux like fingerprint inclusions with higher reflections.