I'm putting in a marble shelf that is going to go in a small inset/nook area, so there are walls in the back and on both sides. The shelf will be 16 inches deep (on the sides) and 30 inches long (it can be anchored in the back but will be open in the front. I'm wondering what would be the best way to support this shelf.

Below it will be some wood shelves (exactly the same dimensions). For these I'm using 1x2s screwed into studs on each of the 3 sides -- will this be enough support for a marble shelf or do I need something more substantial (I've never installed marble)?