Modern methods of gemstone modification

By Olga Demina gemstones expert at Lithuanian Assay Office

Modifications (in the Russian language – beautification; in English, this term is called treatment – color and quality improvement) are special processing procedures, not including polishing, finishing and cleaning, that change the appearance of the stone, such as color, transparency, and optical effects (for example, creating a star effect in rubies). In addition, modifications are used to increase the wear resistance and durability of stones. Recently, they have also been used to simplify the process of extracting gemstones from rock, to make it easier and less expensive to enrich the raw material.

Gemstone modifications are often applied to make it possible for raw material, that was originally not of gemquality, be transformed from waste and low-quality initial product which does not participate in the jewelry market of gem-quality raw materials, by adding transparency, improving color quality and enhancing durability characteristics. In other words, thanks to modification, it is possible to give low-grade raw materials all the characteristics of gemstones. According to the classification of English mineralogist J. F. G. Smith, precious materials are those that best meet the three main criteria: ‘beauty, durability, and rarity.’

Lately, there has been a tendency of increasing interest in natural stones. For example, the fashionable nowadays craze for microphotography offers scope for a new hobby related specifically to mineralogy and the world of gemstones. If some time ago we could admire only the external beauty of the stone and only a professional with an expensive lens could peek into the rich world of precious stones, today, with the reduction of the price of optical devices (at popular Internet sites you can buy a minimicroscope for your phone for just a few euros), we are also able to access this incredible world. And get great pictures of inclusions in minerals.

That is why there are more and more suggestions that all stones of natural origin (which meet the criteria of beauty and durability) should be categorized as precious, since the limiting factors lead to the rarity of these materials and thus historically and culturally most consumers pay attention to the origin of the stone. This is an important selection criterion.

Synthetic stones and materials created in laboratories are one more segment of the jewelry market. It will also develop and grow but buyers of these segments are often different, with different motivations and criteria in their choice of jewelry.

So why has the number of modified stones increased on the market most recently in particular?

There is an opinion that this is the intention of the market to create a balance of supply and demand. It’s due to the recent excavation and depletion of easily accessible gem deposits. Either most of today’s large deposits are located distant from the market of demand – in this case, transportation costs are imposed, or excavation is already difficult: it is no longer an open pit excavation, and it is necessary to create mines. Also, excavation methods are influenced by the environmental factor, i.e. the negative impact of mining on nature and the environment. This also increases the cost of the final product.

Though the demand for precious stones experienced a decline in some regions due to the crisis, it still remains at a fairly high level.

This is why modified natural stones are a good way out for the gemstone market. Both natural stones and stones created in laboratories are subject to modification but at the moment, they are the modified natural stones that are widely represented and occupy a visible segment of the jewelry market. Since the exhaustibility of natural resources is a limiting factor, there is a ‘cost-quality’ balance.

STONES OF NATURAL ORIGIN

Combination of two factors: especially rare specimens, high price

Stones of good quality characteristics and sought-after colors

Combination of two factors: low quality, unappealing color

Combination of three factors: modified natural stones

Combination of two factors: synthetic stones and artificial materials

Opportunity to buy on the mass market, reasonable prices

Filled crack in Emerald

Diamond with filled cracks

In other words, as it is demonstrated in the scheme, modified gemstones (jewelry) are a combination of three desirable factors such as genuineness, rather good characteristics of transparency and durability of the stone, i.e. quality and popular attractive color, but also the cost of such stones remains affordable to the mass market.

Natural jewelry stones with good characteristics of transparency, flawlessness and desirable fashionable colors are quite expensive and often become luxury items when it comes to large jewelry inlays.

As for small natural stones of mediocre quality, their attractive effect compete on the market with synthetic stones, which often cost less and look more attractive. But they are not shrouded in a halo of uniqueness and individuality and do not have inclusions that were formerly called no other way than defects which are today more often called peculiarities that confirm the natural origin of a stone. If previously the certificates of the world's laboratories were limited only to a scheme of inclusions-defects, now some gemological laboratories issue special photo albums with the inner world of natural stones, this world is often unique and interesting. Inclusions of different other minerals or inclusions of unusual shape only increase their value, making such stones a unique commodity and collector’s item.

Thanks to the development of science and technology, there are today quite a lot of methods for modifying jewelry raw materials. Often there are combinations of the following:

● Heating (high/low temperature) – today is considered a routine procedure to improve the transparency and color of gemstones. The techniques range from conventional or microwave heating up to the application of highly sophisticated processes using gas equipment and pressing machines. ● Annealing is a type of heat treatment that involves heating to a certain temperature, exposing to that temperature for a certain amount of time, and then slowly cooling to indoor temperature. Annealing involves the processes of regeneration, recrystallization and homogenization, which reduce the stresses in the crystal lattice. This is necessary for a higher yield of stone suitable for processing. ● Filling cracks – with natural oils (considered an acceptable practice of soft upgrading), filling with various resins, polymers, wax, paraffin, etc. ● Impregnation (stabilization) is application of colorless or colored waxes or similar materials, pouring polymers over the entire volume of the stone or on the surface layer of the stone. ● Crack healing with glasses (for diamonds) and glasses with lead or cobalt impurities (for Rubies, Sapphires). ● Healing cracks in Sapphires and Rubies with flux agent.

Flux is a natural or artificial substance that is used to lower the recrystallization temperature and consequently the cracks in the stone heal. Flux is most often a multicomponent mixture with the predominance of Borax (Sodium tetraborate decahydrate Na2B4O7·10H2O) ● Dyeing and cracking followed by painting. It’s mostly applied to quartz. Very strongly cooled stones are placed in a very hot solution (liquid), due to cracks (a whole system of cracks is formed), the stone is colored and acquires a specific, initially targeted color. ● Diffusion

Green-colored Quartz imitating Emerald

Diffusion is a nonequilibrium process under which a substance moves from a region of high concentration to a region of low concentration, resulting in spontaneous equalization of concentration throughout the occupied volume. The example of it is the effect of optical asterism in rubies and sapphires. A fabricated cabochon is coated with a titanium film, then heated, and the rutile phase crystallizes along its crystallographic faces and directions, resulting in an optical asterism effect. But the depth of diffusion is usually up to 0.5 mm.

● Modification with X-ray and gamma-ray irradiation. It’s very often used for Topaz, Diamond, Citrine and Smoky

Quartz. ● Metal-coated crystals – such method is used, for example, for Quartz whose surface is coated with metal to give it a shimmering metallic luster. Coatings often include Gold, Indium, Titanium, Niobium, and Copper. ● Bleaching is the changing or lightening of color through exposure to physical or chemical agents, or through the use of light. ● Reconstructed stones are artificial materials produced by melting, without subsequent crystallization or pressing of natural materials, into one integral whole. ● Laser drilling is drilling out channels and holes in a stone with a laser in order to remove inclusions from this stone. ● Gluing is joining of two or more parts into one integral whole with the help of glue or some other chemical substances (creating duplets, triplets).

Today, with the appearance of such a great number of imitations and modifications of gemstones, it is especially important to retain the trust of the buyer. Therefore, an obligatory condition for the sale of modified gemstones is the disclosure of complete and accurate information about the modification. This also complies with the rules established by CIBJO international organization (The World Jewellery Confederation). However, with one reservation: it is possible to omit information about the modification on the condition that this procedure is generally accepted. And today, more and more types of modifications are becoming routine procedures, such as heating for Sapphires or Tanzanites.