O.K., so the "very, very" is stretching it a bit. But "rock" is a good insulator;

                       W/(m.K)
Asbestos-cement board  0.744
Brick dense            1.31
Fireclay brick @ 500C  1.4
Granite                1.7 - 4.0
Gravel                 0.7
Mica                   0.71
Limestone              1.26 - 1.33

http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

Earth formed from a collision
www.preearth.net

Plate-tectonics is wrong
www.preearth.net/plate.html