What is "foil stone"

Foil \Foil\, n. [OE. foil leaf, OF. foil, fuil, fueil, foille, fueille, F. feuille, fr. L. folium, pl. folia; akin to Gr. ?, and perh. to E. blade. Cf. Foliage, Folio.]

1. A leaf or very thin sheet of metal; as, brass foil; tin foil; gold foil.

2. (Jewelry) A thin leaf of sheet copper silvered and burnished, and afterwards coated with transparent colors mixed with isinglass; -- employed by jewelers to give color or brilliancy to pastes and inferior stones.
   --Ure.

3. Anything that serves by contrast of color or quality to adorn or set off another thing to advantage.

   As she a black silk cap on him began To set, for foil of his milk-white to serve.
   --Sir P. Sidney.

   Hector has a foil to set him off.
   --Broome.

4. A thin coat of tin, with quicksilver, laid on the back of a looking-glass, to cause reflection.

5. (Arch.) The space between the cusps in Gothic architecture; a rounded or leaflike ornament, in windows, niches, etc. A group of foils is called trefoil, quatrefoil, quinquefoil, etc., according to the number of arcs of which it is composed.

   Foil stone, an imitation of a jewel or precious stone.

n. (context obsolete English) An imitation jewel or precious stone.