aventurine is a type of glass cooked with copper shavings and left to cool in the pot. It comes out of the oven as a single lump and is cut to blocks for further processing.
In order to use it in a larger piece it needs to be picked up on the marver as shards, but it cannot be worked too extensively after that because subsequent reheating will burn the copper flecks. Aventurine can only be worked at relatively low temperatures - so onlly at surface, never inside a thickwalled piece.
It is also used as threads in reticello glass - which is a relatively low temperature production. I think Barovier managed to make small glasses out of solid aventurine glass.
Gold leaf/ gold foil (same thing) and silver can be used in thickwalled pieces because they do not burn.