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Gemstone Identification & Classification Flashcards

7 cards from real CGC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 7 Gemstone Identification & Classification flashcards as text

  1. A colorless stone reads RI 1.544–1.553, birefringence 0.009, and SG 2.65. What is it most likely?

    Answer: Quartz

    These RI, birefringence, and SG values are diagnostic for quartz.

  2. Garnets crystallize in which crystal system, making them singly refractive?

    Answer: Isometric (cubic)

    Garnets are isometric, so light travels at one speed and they are singly refractive.

  3. Under a polariscope, a stone stays dark through a full 360° rotation. What does this usually indicate?

    Answer: Singly refractive or amorphous

    Remaining dark in all positions indicates a singly refractive or amorphous material.

  4. Which instrument is used specifically to observe pleochroism in a gemstone?

    Answer: Dichroscope

    A dichroscope shows two pleochroic colors side by side in doubly refractive stones.

  5. Which gem gives an over-the-limit (OTL) reading on a standard refractometer?

    Answer: Cubic zirconia

    Cubic zirconia's RI of about 2.15 exceeds the standard 1.81 contact-liquid limit.

  6. Strong doubling of the back facets in a near-colorless diamond simulant points to which material?

    Answer: Synthetic moissanite

    Synthetic moissanite is doubly refractive, so facet doubling is visible, unlike in diamond.

  7. 'Horsetail' inclusions of chrysotile fibers are a classic identifier of which gem?

    Answer: Demantoid garnet

    Radiating chrysotile 'horsetails' are characteristic of demantoid garnet, especially from Russia.

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