It consists of a lot of hard-compressed talc, with crystalline veins of harder material.
Its really nice to work with; much of it done with common carpentry tools with masonry blades, carbide bits, etc. Its quite machinable with common hand tools. My fanciest tools are a wet tile saw (table saw style), and router. Beyond that, common hand drills, angle grinders, machine drill bits, wood spade bits; actually, wood spade bits work really well!
I'd have to compare it to working with the more sandy of flagstone, like the arizona stones.