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A thin diamond with two large, flat, parallel surfaces so that the viewer can see through it to recognize an object under it, without distortion by refraction. The largest known is the 'Russian Table Portrait Diamond', 25 carats, 4 by 3 cm, now in the Kremlin, Moscow; it is thought to be a cleavage from a larger stone. Sometimes such a diamond is found in nature (called a 'flat') and is cut and bevelled for use as a glass over a miniature portrait or for a watchcase (called a 'portrait stone' or a Lasque).
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson