Goldstone is a rust coloured glass. The glittery flashes are made of copper. When copper is added to molten glass, the copper crystallizes. Interestingly, even though goldstone is man-made, you will often find it listed with gemstones. It was discovered in the early 1800's, by an Italian monk who was making glass at his monastery, when he accidentally spilled some copper filings into his molten glass and goldstone was born.