Treatment and Imitation Pearls

Treatment means any action by man (other than polishing, cleaning, buffing, peeling, bleaching, drilling, cutting, and working) that alters the appearance of a pearl or cultured pearl.

Methods of Treatment

Bleaching

The Japanese harvest is certainly always beached and the same is true of the Chinese Akoya production. Bleaching is an age-old method which was also applied to natural pearls.

The bleaching process changes the color pigments contained in the organic substance. The time required is between seven to sixty days, and it is applied until the pearls have reached a uniform white color. A period of more than thirty days may prove damaging to the pearls, as they may begin to show cracks due to the progressive desiccation of the conchiolin substance.

Pink Coloration

About 95% of all pearls are treated with a coloring agent after the bleaching process, producing a light even hue and a more or less pink overtone.

The pink coloration is more or less accepted in the trade as long as it is permanent, does not look artificial and can’t be recognized at the drill hole or the surface of the pearl, and it is usually not mentioned when the pearls are sold.

Polishing

After processing, the pearl factories usually polish the pearls in a drum by adding stripes of cork or tiny pieces of walnut shell or bee’s wax may be used. Polishing takes two to three hours with 20 to rotations per minute to enhance the luster of the pearls. Larger pearls are sometimes polished on felt wheels using a mild polishing agent.

Dyeing with Silver Salt

Treatment with silver nitrate produces a black color. The method was used at the end of the 19th century in the mother-of-pearl industry and natural pearls were also dyed this way.

The pearls are immersed for a period of several days or weeks to several months in diluted silver nitrate solution and are kept in a dark room. After this, they are exposed to a strong light source (sun light or artificial light) or they are treated with hydrogen sulphate. Both methods lead to the deposition of metallic silver in a fine distribution in the pearl, causing the dark color.

The methods enable the production of lighter or darker hues, depending on the quantity of silver produced. An immersion period of several months is required in order to produce a truly black color while shorter periods lead to more brownish or grayish hues.

The metallic silver precipitated during the process is opaque to X-rays and may be detected by the presence of fine white lines on the photographic negative.

Irradiation

The gamma ray irradiation changes the color of the nucleus to a more or less dark hue, while the outer pearly layer remains unchanged. Irradiated pearls appear silvery grey to grayish brown, but black is rarely encountered.

Imitation Pearls

White beads made from mother-of-pearl are one of the oldest types of imitation. The beads can be recognized with the naked eye due to their layered structure.

Other imitation pearls may be beads of glass or plastic, coated with a material known as guanine. The guanine is obtained from herring fish scales, where it occurs as tiny crystals embedded in tissues attached to the scales.

An older type of imitation pearl consisted of a hollow glass bead, the inside surface of which was coated with ‘essence d’orient’. The bead was then filled with wax.

Both of ‘conch pearls’ and ‘tridachna pearls’ can be easily be distinguished by its striped or banded structure, and absence of the ‘flame structure’.

Under a 10x lens, the surface structure of imitation pearls has a smooth to somewhat granular appearance, which contrasts markedly with that of natural and cultured pearls.