Cavity and Fracture Filling
Some stones are fragile in their natural state, with cracks along the surface dulling their luster and muting color. Some imperfections are large enough to threaten a jewel’s structural integrity. To improve their appearance and add strength, some manufactures may fill in these cracks with colorless substances. When the blemishes are wide, the process is called cavity filling, while fracture filling describes the treatment of narrow fissures.
The material used to fill in these gaps range from plastic, glass and shellac to oil. Some stones have a filler of choice, such as cedar oil for emeralds and lead rich glass for rubies. Depending on the size of the blemish and the material used, the treatment may be obvious, shining differently from the surrounding stone. Manufacturers aim to make fracture filling as unobtrusive as possible. If you want to know if a piece of jewelry you’re admiring has this treatment, look for a disclosure notice in the display case, or ask a staff member.
The process of using heat to transform gemstones is an ancient one, dating as far back as ancient Egypt. High temperatures usually modifies a jewel’s color, either lightening, darkening, saturating or completely changing the hue. Other times, heat treatment alters inclusions within the stone, making the jewel eye clean or enhancing desirable clarity characteristics. The technique can be complex, using a variety of temperatures, pressures, levels of oxidation and more to get the desired result.
Stones such as sapphires, tanzanite, and citrine are commonly treated to improve their color. Almost all examples of these jewels on the market have been treated, as ones with naturally occurring desired hues are quite rare. Heated jewels are usually stable, with colors that won’t fade or darken with time. Certification and appraisals can help you determine if your stone has been treated and in what way.
Just as bleaching is used to improve the color of paper and clothes, bleach is sometimes used on gemstones to enhance light hues. Pearls often receive this treatment to create a uniform appearance and to remove uneven coloring. Other jewels to get this treatment include ivory, jade and some forms of chalcedony. Depending on the gemstone and the color that needs to be removed, strong lights may be enough to bleach an item. Other times chemicals such as hydrogen peroxide and chlorine bleach are used.
Sometimes bleach is combined with other techniques, such as drilling a thin hole into a diamond in order to lighten an inclusion. This treatment should be used with care, as to hide one clarity characteristic, a new one in the form of the drill mark is created. Even when bleaching is confined to the surface of the stone, it’s best to use caution to prevent weakening the jewel. In most circumstances, the end result is a gemstone that is all the more beautiful.
Just like clothing is dyed to provide a wider range of colors to delight the eye, gemstones may also be dyed to offer a rainbow for people to enjoy. For a jewel to retain additional coloring, it needs to be porous. Gemstones that are naturally able to take on dye include pearls, coral, turquoise and jadeite.
Non porous stones require preliminary treatment before they are able to accept dye. One technique involves rapidly heating and cooling a stone, causing cracks to form, a process known as quench crackling. The coloring agent seeps into the cracks, holding the dye in place. This variation of dyeing can be used to create simulants, such as coloring rock crystal to mimic a ruby.
In gemology, irradiation refers to bombarding jewels with subatomic particles or electromagnetic radiation to enhance or alter a gemstone’s color. If used correctly, irradiation skews a stone’s atomic lattice structure, changing its color. Gemological irradiation dates back to 1904, when scientist and jewel connoisseur Sir William Crookes buried a diamond under radium salts. The surface of the stone turned dark green, but was too toxic to wear.
Today irradiated jewels are tested to meet strict safety guidelines before they’re released to the market. Topaz is frequently irradiated to produce a light blue hue rarely found in nature. Pearl, quartz, diamond, beryl and tourmaline may also change colors through this treatment. New developments in gemstone irradiation are tested for stability before being used in broad applications.
One way of changing the color of corundum is through lattice diffusion. The stones are heated to just below 2,030 degrees Celsius, the melting temperature of sapphires. Chemicals are introduced to the corundum, which penetrate the crystals’ softened surfaces. Iron and titanium are used to make blue, chromium red, and beryllium a rainbow of hues. Bland corundum becomes vibrant, and rare shades becomes more available. With the right chemicals and cooling speed, these treatments may even induce asterism.
The high temperatures allows atoms from the outside material to bounce into and around the lattice structure of the corundum. This is not unlike placing marbles on a peg board. While the pins remain in place, the loose pieces travels around the structure.
Sugar and Smoke
To bring out the play of light in an opal, a dusky body color is ideal. Dark shades brings the prismatic light inside the stone into relief. For this reason, black opals are highly prized, but are also rare. One approach to highlighting the beauty of opals with white body color is to place thin slices of the material onto ironstone or other dark stones, an arrangement known as doublets. Other techniques are called sugar or smoke.
The former involves placing an opal in a sugary fruit juice solution. The liquid is heated, helping the substance to sink into the opal’s porous surface. After the stone cools, it’s soaked in sulfuric acid, carbonizing the sugar. Smoke treatments may consist of roasting a paper wrapped opal, or burying it in a pot full of dung and baking it. All three methods darken the surface of the jewel.
As the name suggests, surface treatments don’t effect the entire crystal, just its upper layers. While actions such as dyeing and bleaching are technically surface treatments, the term usually refers to three variations known as coating, backing and painting. Unlike processes like irradiation and lattice diffusion, surface treatments date as far back as 2000BCE.
Backing a stone involves placing a bright or colorful item such as foil or paper behind a jewel to enhance its brilliance or hues. They’re still used today in the form of doublets and certain mounts that place a mirror-like polish behind the jewel. Modern backings are more common in fashion jewelry than in fine. Coatings involve covering a stone in varnish, wax, plastic or other materials to alter or improve its color, and painting uses paint or ink in order to change color. These last two treatments are fragile and may come off of a jewel. If you have bought a stone with these kind of treatments, take special care to avoid damage.
Ideally, treatments on a gemstone are made to last. Like jewel durability, the best treatments won’t fade or come off with rough handling. However, gems can be longer lasting than the processes made to make them prettier. With the right care and knowledge, the connoisseur should be able to keep their jewels pristine.
Less stable treatments are why it’s best to clean jewelry with the most gentle method possible. Ultrasonic cleaners or boiling water may dislodge fracture fillers such as the oil in an emerald, or the coating of a simulated gemstone. Likewise, too much exposure to sunlight may cause some irradiated stones to fade. Wearing a ring or bracelet decorated with modified gemstones while cleaning or bathing could also harm treatments, such as staining a bleached jewel, or leave discolored patches. No matter the provenance or journey one’s jewelry has experienced, caution is the best way to keep it beautiful.