From Wikipedia: Goldstone is a type of glittering glass made in the presence of a reducing flame. The finished product can take a smooth polish and be carved into beads, figurines, or other artifacts suitable for semiprecious stone, and in fact goldstone is often mistaken or misrepresented as a natural material.
The most common form of goldstone appears to be reddish-brown, due to the presence of tiny crystals of metallic copper. When the reduced goldstone melt cools, the copper remains in atomic isolation and precipitates into small crystalline clusters which reflect light through the surrounding transparent or translucent glass, rendering the final product effectively opaque. (Under normal oxidative conditions, copper ions meld into the silica to produce transparent bluish-green glass.) Some goldstone variants have an intensely-colored glass matrix—- usually blue or violet, more rarely green—- and a more silvery appearance to the suspended crystals, whose color may be partially masked by the glass, or which may be based on different metals (perhaps cobalt, manganese, or chromium).
The manufacturing process for goldstone was discovered in seventeenth-century Venice by the Miotti family, which was granted an exclusive license by the Doge. Persistent folklore attributes the discovery and secret of goldstone to an unnamed Italian monastic order, giving rise to the alternate names "monk's gold" or "monkstone". Another name, "stellaria", is based on the starry internal reflections. It is also called "sun sitara".
Curiously, goldstone is one of the few cases where a synthetic simulant provided the eponym for the similar natural stones. The original Italian name for goldstone is "avventurina" or some similar word or phrase indicating its accidental discovery, hence the mineral name "aventurine" for forms of feldspar or quartz with mica inclusions that give a similar glittering appearance. The technical term for this optical phenomenon, "aventurescence", is also derived from the same source. Yet another name for goldstone is "aventurine glass", but this should be discouraged to avoid confusion with the minerals.