Insulating bricks, the light one with lots of little air holes, would make a good insulating layer under the hearth. I wouldn't go shopping for the specifically for that purpose, as there are modern insulators that are much better, but if you have them -- go for it.
For fun, take the How do They Make Insulating Firebricks quiz:
http://www.fornobravo.com/forum/f6/h...icks-1077.html (How they make insulating firebricks)
James