[Rockhounds] when volcanoes drop bombs: giant crystal payload

[Richard]

I like your explanation, Pete.  Considering the temperatures and pressures 
that these formed, is it possible that either the chemistry and/or crystal 
structure quickly became unstable at the surface and the crystal lost 
transparency from the inside out...  thus appearing to 'grow' to an excited 
observer?

Rich

 >Those crystals are "megacrysts", also could be called  "xenocrysts".  They
> grew somewhere else when the magma was still deep down,  10s of kilometers
> deep, and then they were carried up by the magma and encased  by a glob of 
> it
> when it was erupted.  Any crystal growth while embedded in  that rapidly 
> cooling
> lava bomb would have been very very minor, on a microscopic  level along 
> the
> very surface edges of the megacryst.