Pearl Glossary

EGL USA’s Pearl Glossary provides an overview of gemological terms related to pearls.
It can be accessed alphabetically, below. Additional information is available in EGL USA’s
main gemological glossary, as well as subject-specific glossaries for diamonds and colored gemstones.

Search by letter




BAROQUE – Irregular and/or non-symmetrical in shape, such as baroque pearls.

BLEACHING – A topical treatment, typically using acids or hydrogen peroxide, that lightens and/or evens out hue. Bleaching is used on select colored gemstones, including jadeite and coral, and pearls.



COLOR/OVERTONE – In reference to pearls, the main color or body color can be white, cream, gray, black, yellow, etc. Often, a pearl will also display an overtone: a translucent secondary color on its surface. Beautiful pearls can be found in virtually any color/overtone.

CULTURED PEARL – A pearl initiated by the implantation of a nucleus (a shell bead or piece of mantle tissue) by a pearl farmer. Although they are more common than natural pearls, cultured pearls — such as the classic South Sea, lustrous Akoya, and colorful Tahitian — can be quite stunning.

Typical Cultured Pearl


Non-Nacre Body

Nacre Coating



DYEING – On a colored gemstone, the application of coloring agents to alter, intensify, or improve consistency of hue. In pearls, the use of chemicals or dyes to change the nacre’s color or overtone. The results of this process can range from distinct color transformations to subtle overtone shifts. EGL USA reports clearly identify dyed colored gemstones and pearls.



ENVIRONMENT – For pearls, the original setting for growth: freshwater or saltwater.



FLUORESCENCE – The capacity of diamonds, colored gemstones, and pearls to emit visible light when exposed to higher energy wavelengths. In diamonds and colored gemstones, this occurs when their atoms react to long- and short-wave ultraviolet rays. In pearls, this occurs when some of their elements react to x-rays. Fluorescence is measured for identification purposes and described on a scale from inert (none) to very strong. Its presence can also help to confirm a pearl’s cultured origin.



GEMSTONE – A mineral, rock, organic, or inorganic material that is, typically, cut and polished for use in jewelry. There are dozens of types of gemstones — including diamonds, colored gemstones, and pearls — each with a unique set of physical and optical properties.

GRAINS – For pearls, a unit of weight measurement equivalent to .065 gram. Depending on pearl type, weight can be measured in grams, carats (.2 gram), or grains.



HEATING – On a colored gemstone, the application of controlled high temperatures to alter or improve its color and/or internal structure. Heat enhances many types of gemstones. And it achieves this by essentially continuing the process started by nature. For example, heat can turn a naturally brown tanzanite into a striking blue/violet, or transform a cloudy ruby to a distinct red. Pearls can also be treated with high temperatures to modify color. However, heat is often used with other treatments, such as bleaching, to maximize effects.

HUE – The primary impression of color such as red, green, or blue.



IRIDESCENCE – Prismatic colors on the inside or surface of a material, caused by light interference from thin layers of differing refractive indices. These layers may be thin films of liquid, gas, or solid. Pearls are best known for exhibiting iridescence.

IRRADIATION – The use of radiation to alter the appearance of a diamond, colored gemstone, or pearl. On a diamond or colored gemstone, radiation such as neutron or electron bombardment can change the internal structure and, therefore, the stone’s color and perceived clarity. This process is often followed by heating, which can further refine the results. Irradiation can create vibrant pink, yellow, green, and green/blue hues in diamonds, as well as beautiful blue topazes and red tourmalines. On certain pearls, such as freshwater or Akoya pearls, gamma ray treatments can generate darker gray, blue, or black colors.



LUSTER – On a pearl, the reflective quality or brilliance of the surface, measured on a scale ranging from poor to excellent.



MATCHING – In pearl strands or jewelry, the coordination of the pearls — in aspects such as color and shape — to create an aesthetically pleasing piece. It is described on a scale from poor to excellent.

MEASUREMENTS – For round diamonds and colored gemstones, an indication of maximum-minimum diameter x depth, in millimeters. Fancy shapes are indicated by length x width x depth. For round pearls, an indication of diameter, in millimeters. Other pearls are measured by length x width x depth. Measurements of pearl strands or jewelry are described as a range, average, or graduation from maximum to minimum.

MOUNTING – The portion of a piece of jewelry in which a gemstone or other object is set.



NACRE – The iridescent coating over a pearl’s nucleus. Its depth can impact the pearl’s luster, durability, and quality. On bead-cultured pearls, thickness is rated as thin, medium, or thick (which is preferred).

NACREOUS PEARL – A pearl that appears translucent, due to its fine, plate-like crystalline layers of calcium carbonate (mostly aragonite) and organic material (conchiolin).

NATURAL PEARL – A pearl created when nacre forms around a microscopic irritant, without human interaction. Such pearls are often coveted for their rarity (as opposed to more commonly available cultured pearls.)

Typical Natural Pearl


Nacre Body

NON-NACREOUS PEARL – A pearl made of coarser crystals (mostly calcite), resulting in a more porcelain-like look than that of a nacreous pearl. Examples include a conch or melo pearl.



ORIENT – On a pearl, the soft iridescent glow on or just below the surface. It is caused by the interference and diffraction of light from the minute crystals comprising the nacre.

OVERTONE – See Color/Overtone.



PEARL – See Cultured Pearl, Nacreous Pearl, Natural Pearl, and Non-Nacreous Pearl.



RAMAN SPECTRA – A representation of the vibrational modes of a diamond’s, colored gemstone’s, or pearl’s molecules as they react to a monochromatic (laser) light source. For diamonds, the Raman spectra measure photo luminescence to detect treatments to the stone. For all stones, Raman spectra are recorded on a graph that plots intensity versus frequency. And they can help identify the species and/or variety of stone, as well as any applied treatments.

REFRACTIVE INDEX – The degree to which visible light bends as it passes through a diamond, colored gemstone, or pearl. Each type of gemstone exhibits a unique refractive index (RI) or RI range — a result of its distinct chemical composition and physical crystallization. As such, RI is a strong identifying characteristic.



SATURATION – An attribute of color that denotes its strength.

SHAPE OF PEARLS – A variety of forms that includes round, off-round, oval, drop (pear-shaped), button (with a flat side), circle (with rings/ridges), and baroque (irregular and/or non-symmetrical).








SURFACE COATING – On a diamond, colored gemstone, or pearl, the application of a thin artificial layer on the stone’s exterior to impact appearance. Coatings are, typically, not considered permanent; their effects may diminish over time.

On a diamond, chemical coatings can disguise less desirable interior hues. On a colored gemstone, coatings can improve color and overall appearance. And on a pearl, coatings can create the illusion of a smoother surface and/or enhanced luster. On an EGL USA report for any stone type, coatings are clearly noted. In addition, EGL USA does not provide color or clarity grades for coated diamonds.



TONE – An attribute of color that denotes its lightness or darkness.

TOUGHNESS – The ability of a mineral or gemstone to resist breakage. Jadeite is the toughest gemstone due to an interlocking molecular structure that helps to minimize its weak points.

TREATMENT – The application of processes or agents to enhance the perceived color, clarity, phenomena, or durability of a diamond, colored gemstone, or pearl. Treatments are routinely applied to stones, with measurable results. Treatments are disclosed on EGL USA reports.

For diamonds, see also Annealing, Feather Filling, High Pressure and High Temperature (HPHT), Irradiation, Internal Laser Drilling, Laser Drilling, and Surface Coating.

For colored gemstones, see also Bleaching, Diffusion, Dyeing, Feather Filling, Heating, Impregnation, Irradiation, and Surface Coating.

For pearls, see also Bleaching, Dyeing, Heating, Irradiation, and Surface Coating.



ULTRAVIOLET LIGHT – Light that is invisible to the naked eye because it consists of wavelengths shorter than those of visible light.



X-RAY FLUORESCENCE – Also known as XRF, the capacity of a diamond, colored gemstone, or pearl to emit energy via secondary x-rays when exposed to a high-energy incident (primary) x-ray. The intensity of the emitted x-rays indicates the stone’s chemical elements for identification purposes.

Note: All photos and diagrams above are intended as general guides.
Renderings may vary with computer settings.