This is a very nice Halley's Comet Brooch. The Victorians were very excited about the discovery of this comet, and fashioned all sorts of jewelry celebrating it. The most popular form was a brooch. This one is very nice, gold, I think, though I haven't tested it, and past. It is circa 1830, and measures a little over 1 and 1/2 inches long, and 1/2 inch wide. It has the original pin and catch.
Paste Halley's Comet Brooch, French, 1830