From the marble-collector side of things, I figure I might add that aventurine -- the sparkly gold glass kind -- is actually a precipitate of copper, produced through a kind of tricky process. Generally it's present in the hot glass as cupric oxide (a colorant, used primarily for blues and greens), but if present in enough quantity and the glass is heated hot enough in an oxygen-reduced atmosphere, tiny crystals of copper will form. It can look like gold or copper. It can also be blue or green, or sometimes bronze or even black or silver. One of the vintage marble companies (Peltier Glass) used it fairly often on certain marble types in the 1930s. Interestingly, if you take it beyond the point of forming aventurine you might get what marble collectors call oxblood.

A Peltier "Zebra" with bronze and green aventurine, and an Alley Agate "oxblood."