Anne, I agree with Jan, it looks like a fine art deco pendant from the photo. Let's think about the marcasite in this antique piece for the moment. True marcasite is almost white, and resembles pale bronze, and was used a very long time ago by the Greeks and the Incas. The substitute Pyrite was used extensively in Europe from the 18th century onwards, especially popular in France. Marcasite is usually set in SILVER and is generally ROSE- CUT or mounted in a pave setting to enhance its sparkle. If it is marcasite, it must be good quality as it has you stumped!
It should be fairly obvious under jewelers loupe, to tell the difference and to see if the stones in the setting are pyrite or diamonds.
a Very lovely art deco pendant indeed! I doubt it would be pearl though because of its square shape. If it feels cold to the touch it could be a stone such as rock crystal. I believe rock crystal was quite popular in the Art Deco era.
