Glossary of Pearl Terms R

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radial structure: Calcite or aragonite crystals of a pearl's prismatic layer are arranged in a pattern, so they radiate like rays of sunlight from a central core. In a cross-section view, the crystals stand on end rather than lie flat as in the nacre layer.

rainbow pearl: From Pteria sterna but may exhibit colors similar to those of pearls from the black-lip mollusk (Pinctada margaritifera).

rakes: Used to collect wild freshwater mussels by dragging across the beds of rivers, lakes and ponds.

Raman spectroscopy: Laboratory method of testing cultured pearls for presence of silver nitrate, which is a pearl dyeing agent.

rati: Historical Indian weight unit used to valuate pearls, equal to 0.91 carats. Also called a poonak.

reborn pearls: Second-generation pearls are harvested and the mussels returned to the water with their pearl sacs intact. They continue to deposit nacre resulting in another pearl.

red tide: Profuse hyperproduction of algae, called an algal bloom. It weakens or kills pearl-bearing mollusks by consuming most or all the dissolved oxygen in the water and/or poisoning them when the algae die and putrefy. It's not a true tide, and it's not always red.

Red Sea cultured pearl: Cultured pearl produced in the local black-lip (Pinctada margaritifera erythraensis) mollusk in the Red Sea.

refractive index: Measure of the degree that light bends when it moves from one medium to another, for example, from air into nacre. The refractive index of natural and cultured pearls is 1.51 to 1.66.

Regente, La: Egg-shaped natural pearl weighing 302.68 grains (75.67 carats).

Reginaia ebensus: Natural pearl producing freshwater mollusk found in the United States, otherwise known as the ebony shell.

Reine des Perles, La: Round natural pearl from the French crown jewels. It weighs 110 grains (27.5 carats).

rice krispies: Chinese freshwater pearls that often have a textured surface and look like puffed rice kernels. They take their name from the American breakfast cereal Rice Krispies, called Rice Bubbles in Australia. The pearls were cultured in the cockscomb mussel (Cristaria plicata) and first appeared on the international market about 1970.

Roman pearls: Imitation pearls also known as "alabaster pearls".

rope: Pearl strand measuring more than 45 inches (115 cm).

rotten: One of four Saxony quality factors used to describe natural German river pearls, rotten being the lowest quality. Also see: light pearls (highest quality), half-light pearls (high quality), and sand (low quality).

round: Perfectly round pearl, or one with diameters that don't vary by more than 2 percent.

royal gem pearl: Imitation pearl produced from the lens of a fisheye.
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