R

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.

red tide

profuse hyperproduction of algae, called an algal bloom, that weakens or kills pearl-bearing mollusks by consuming most or all of the dissolved oxygen in the water and poisoning them when the algae die and putrefy

refractive index

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

rope

pearl strand measuring more than 45 inches (115 cm)

round

perfectly round pearl, or one with diameters that don’t vary by more than 2 percent