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layered stone
consisting of two differently coloured layers of agate. as naturally layered stones are extremely rared most layered stones are glued. used as seal stones for gemstone engravings

loupe clean
a term only used for diamonds. a diamond is called loupe clean when a trained expert cannot detect any inclusions with the help of an aplanatic, achromatic 10x loupe. diamonds with inclusions of a size of more than 5 microns must not be called loupe clean (including outer features e.g. "frazzles" at the girdle).

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