here's some TC data for quartz in W/m/C
from
D.W. Lee and W.D. Kingery, J. American Ceramic Soc., v43, p594 (1960) and G.E. Childs, L.J. Ericks and R.L. Powell, NBS Monograph 131, p514,527 (1973)
1.0985e-001 -268.1
1.5081e-001 -258.1
1.9852e-001 -248.1
2.5175e-001 -238.1
3.0939e-001...