Stabilisation of gemstones

Besides the methods to enhance the color of a gemstone, the enhancement to cover fractures is second in importance.

For example emeralds have been treated with different oils for a long time. The best known is cedarwood oil or also a natural resin called canada balsam. The probability that an emerald is treated is very high. Inclusion or fracture free emeralds are extremely rare. The treatment is very simple, has tradition and promises a higher gain. The treated emerald, appears better in quality than he would be otherwise.

The refrative index of these oils is lower than the refractive index of emerald. This is the reason why the filled fractures are still visible under a microscope, depending the illumination.

Refractive index for
Cedarwood oil       1.512
Canada balsam       1.520
Opticon       1.545
Emerald / Beryl     1.577 - 1.583

For considerable time epoxy resins are also in use to fill or cover fractures. Their refractive index is with 1.54 (Opticon) mutch closer to the refractive index (RI) of emerald. Untreated fractures in an emerald, who are easily visible for the naked eye, become considerably less visible after the treatment and may not be visible with the unaided eye.

The epoxy has many advantages compared to the oil. It can be hardened and therefore does not leak out when the stone is re-cut or cleaned. The hardened epoxy makes a stone more durable and the treatment is also called stabilization.

A good example is turquoise, which is found sometimes in very unstable qualities and most likely will break in the hands of a lapidary if not stabilised beforhand with epoxy resin. An other reason for the treatment is the fact that turquoise chages its color to green when in contact with the acids or oil of the skin and or perfume.

The resin seales the porous structure and surface ot turquoise and so the precious color is protected. A side effect is the higher weight resulting from this treatment. As you mostly pay by weight this adds to the gain of the seller.

This treatment is quite easy to detect on rough turqoise, as the resin leaves traces in surface cavities where they can not be scraped off. As scraped surface is therfore a first indication of treatment. When cutting natural turquoise there should be no odor. If it is stabilized, then a strong smell of burning plastic will appear. Once cut and polished the stabilization is harder to detect. With a loupe you search the surface for epoxy filled cavities.

Besides emerald and turquoise there are other gemstones which are sometimes treated with epoxy; quartz, amethyst, aquamarin, tourmalin and others. Important is the refractive index of the treated stones. The closer the respectiv refractive indexes are the more difficult is the identification of the treatment.

According international directives the treatment must be declared sponanous at the point of sale. This of course is rather an unfullfilled dream. The cleaning of fracture filled gemstones, especialy the ones treated with oil, makes the fractures again visible to the unaided eye. This is especcially annoying if the jeweller or gemstone setter can not identifiy the threatement in advance and inform the customer. He comes under suspicion to have damaged the gemstone or even to have switched it against a less valuable stone. It is easy to imagin the resulting damage of reputation.

Hubert Heldner September 1995

Filling substances for clarity enhancement of emeralds

Natural substancesRI
Mineral oil       1.478
Paraffin oil       1.478
Sesam oil       1.474
Palm tree oil       1.473
Paraffin wax       1.520
Cedarwood oil       1.509 - 1.520
Canada balsam       1.521
Clove oil       1.531
Cinnamon oil (not reported)       1.589

Artificial resins or polymers

Norland Optical Adhesive type 65       1.501
Norland Optical Adhesive type 65 (cured)       1.529
Permasafe       1.565
Super Tres       1.570
Araldite 6010       1.572
Epon 828       1.575
Opticon Resin 224       1.550
Opticon Resin 224 (cured)       1.580

Gems & Gemology Summer 1999, vol XXXV, page 83
On the identification of various Emerald filling substances
By Mary L. Johnson, Shane Elen, and Sam Muhlmeister




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