Published: June 2012
Most natural gems vary greatly in their range and intensity of color even within the same stone. The most desired colors are never available in great enough quantity to meet demand, so a variety of methods to improve color have been used for thousands of years. These color enhancement treatments will vary, depending on a material’s ability to absorb or adhere to a coloring agent. Many gems, such as diamond, will not accept a dye and must be enhanced by coatings or other methods. Some materials, such as shell, ivory and turquoise, are porous and commonly color enhanced. The ease with which these porous materials accept pigments probably made them the first gems to be treated. Other materials, such as lapis, have some areas that accept color more easily than others. Any stone that has cracks running through it can usually be dyed. In some cases, cracks are induced to allow dyes to penetrate.
Dyeing is the oldest and most common color enhancement. Many chalcedonies are seen in vibrant greens and blues, dyed by methods that have been used for hundreds of years. These dyes can vary greatly from organic or vegetable dyes to chemical salts and natural pigments. Some treated gems are obviously artificial in their appearance, but others convincingly imitate the natural colors found in nature. Some dyes may be rinsed away, or fade with time or exposure to light. The best methods of dyeing are permanent and stable. There is rarely just one method of dyeing for a given stone, but it will vary by source, vendor and the technologies available at the time. Dyeing gemstones may be simple in concept, but the details will vary greatly.
Some stones may be simply soaked at room temperature for an extended period until the dye slowly seeps into the material. Pearls are often dyed in this way or with low heat. Natural pigments, chemical salts and heat may speed the process or yield more permanent results. Dyes will often vary in their degree of penetration from just the surface to deep within a gem, so re-polishing is not recommended on most dyed gems. Care must be taken when cleaning such gems, as dyes can sometimes be removed. Dyes must always be disclosed, as all are not permanent and they can greatly affect the apparent quality and value.
Examples of stones that undergo such treatment are the quench-crackled and dyed quartz. The quench crackling allows the dye to enter and color the quartz. The process also creates fissures which help make a convincing emerald imitation. Without the cracks, the dye could not penetrate the quartz. Howlite, a whitish variety of quartzite, is also commonly dyed. The white body color and porosity of the material allow for ideal dyeing conditions. It is quite common for jades to be colored with traditional vegetable dyes and this is often combined with a polymer treatment (B-Jade). Coral is commonly dyed to look like its rarer salmon to red colors. Even expensive gems like emerald, ruby and sapphire, if they have fissures, can have colored oils added to improve appearance. While many of these treatments have been done for centuries, it is important to disclose the presence of dyes and the durability of the added color.
Sugar and smoke do not sound like typical dyes, but they are used to dye some types of opal and chalcedony. These types of color enhancements are sometimes referred to as staining, as they do not use what are traditional dyes. Matrix opal, which is composed of grains of opal within a host rock, can have porous, non-opal areas. These areas, when blackened, can greatly enhance the fire of the opal by providing a dark contrast. By soaking the material in a solution of sugar, then subjecting it to acids (which does not hurt the opal), all but the carbon element in the sugar is removed. This creates a dark, black background. The result is a darkened outer layer against which the opal’s play of color is more dramatic. The darkened layer is usually just a few millimeters thick and can be polished or chipped off the stone. Black onyx, a type of chalcedony, is also treated in this manner, but the penetration is usually quite deep, often throughout the material. In the process of smoke treatment, hydrophane opal is exposed to concentrated smoke, gradually darkening the body color as it permeates inside the opal. Tea is commonly used to give an aged look to ivory and bone. The material is simply soaked in a strong tea until the desired result is reached.
Bleaching is also a color enhancement; however, it removes color rather than adding it. Gems that are commonly bleached include pearls, ivories and corals. In many cases, a mild hydrogen peroxide formula is all that is needed to even the color and make a material appear whiter. Bleaching may be applied before dyeing to even out a gemstone’s color, so that dyes may be applied more uniformly. Some tiger’s eye is soaked in a mild chlorine bleach to lighten the color. These bleaching treatments are stable and permanent.