CGA Colored Gemstones & Pearl Evaluation Flashcards

What is the primary factor that distinguishes a natural pearl from a cultured pearl?

Answer: Whether a human-inserted nucleus was used to stimulate nacre production

Natural pearls form entirely without human intervention, while cultured pearls are produced by inserting a bead nucleus or tissue graft into a mollusk to stimulate nacre deposition around it.

What is 'nacre' in pearl grading?

Answer: The iridescent calcium carbonate layers that form a pearl

Nacre is the iridescent material composed of layers of aragonite crystals and conchiolin that mollusks secrete around an irritant or nucleus, forming the pearl.

Which type of pearl is most valuable and commands the highest prices in the US market?

Answer: South Sea pearls

South Sea pearls, produced by Pinctada maxima oysters in Australia, Indonesia, and the Philippines, are the largest and most valuable cultured pearls due to their size (10–20mm), thick nacre, and satiny luster.

What does 'orient' mean when evaluating pearl quality?

Answer: The iridescent play of color across the surface of a pearl

Orient refers to the iridescent shimmer of colors (pink, green, blue) that plays across the surface of high-quality pearls, caused by light interference within the layers of nacre.

A CGA appraiser is valuing a strand of natural pearls. What instrument can help distinguish natural from cultured pearls?

Answer: X-ray or endoscope

X-ray examination reveals the internal structure of a pearl; natural pearls show concentric growth rings throughout, while cultured pearls typically show a large nucleus with a thin nacre coating.

What is the main characteristic that makes alexandrite valuable and distinctive?

Answer: Its dramatic color change from green in daylight to red in incandescent light

Alexandrite is prized for its dramatic color change phenomenon — appearing green to bluish-green in daylight and red to purplish-red under incandescent light — due to the selective absorption of light by chromium.