Aventurine is not a powder, it is a glass. You can probably buy it in frit form, or by a rod and use a hammer.
Silver or gold leaf can also be used but only fracture when you expand it , with slumping this is not very likely. I am sure you could experiment with copper, or other metal filings, but make sure it has a higher melting temperature than the glass and wear goggles and breathing mask in case of poisonous fumes and flying bits of metal. If you can soften the surface of the glass then you should be able to embed the metal. Bits of copper, or iron etc., have the added advantage, if they stay exposed to atmosphere of oxidising for interesting after-effects. You can get chemicals to create near instant oxidation - verdi-gris (spelt right?) on copper (Pale green).