In the RW, glass is surprisingly easy to discover. IIRC, Neolithic
hearths in the Orkneys had accidentally produced glass because they used
seaweed as fuel. The combination of sand + heat + soda (from the
seaweed) produced glass.
In Glorantha terms, sand is rock that has been defeated (pulverised) by
Water. Perhaps the application of Fire liberates the rock from the
Water, so it can be whole (a solid glass object) again?