See Death at Midnights post "Ice that Burns" 

from wikipedia *
At higher pressures, methane clathrates remain stable at temperatures up to 18 °C. The average methane clathrate hydrate composition is 1* mole of methane for every 5.75 moles of water, though this is dependent on how many methane molecules "fit" into the various cage structures of the water lattice. The observed density is around 0.9 g/cm³. One liter of methane clathrate solid would therefore contain, on average, 168 liters of methane gas (at STP).

I squeezed this out of Bryce.

Ice chunk two identical objects.  One nested inside the other.  The outside uses fuzzy material,  the inner one is normal.  Render time was 12hrs just to plop render the  cube.