Wood .In fact Birchwood .
The skid is made from dried veneers of birchwood laminated with resin to a compact structure.
the trade name for it is Jabroc:
http://www.jabroc.com/
So Jeffsvilleusa your turn as you found the right direction ,matt 21 I hope this is ok for you..even if your answer was perfectly covering what I was expecting .
the Jabroc thing I find very funny in the formula 1 context with all things so precisely engineered only to come up with a skid block made from wood with variing densities ...
On a sidenote -Frank Costin (brother to Mike Costin /Cosworth,build whole chassis from wood and his work is sure worth taking a look at....