It's G10, a fairly tough and useful, yet light weight material. Here's Benchmade's info on it:
http://www.benchmade.com/about_knives/materials.asp
FWIW, the scales on my mini grip feel much, much cheaper than the scales on my 530. I'd like to find some replacements...
The Benchmade 2008 Catalog says this about the 530's handle:
Noryl GTX® 830 Scales; Double 420J Liners; Removable Steel Carry-ClipHANDLE SPECS