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 TablePortrait 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 andbevelled for use as a glass over a miniature portrait or for a watchcase (called a 'portrait stone' or a Lasque).