Here are my new tools for cutting marble: the hardie, a steel chisel, embedded in a block of wood, and the steel hammer. I couldn't find a 6x6 block of wood, so I came up with the brilliant idea of gluing several pieces of 2x6 together. Seems to work fine, though it's not as attractive as a solid block would be. I'll keep my eyes open for a scrap.
As my dear friend Margo warned me, there is a steep learning curve for cutting marble. I'm shattering about 20% of the pieces I'm cutting -- which are the 9/16-inch squares, the only mosaic tiles of colored marble that I could find in small quantities. I'd love it if all of them were the 3/8-inch cubes, but those seem to be in short supply. Also, I'm learning that different marble types behave differently when struck: The off-white called crema marfil cuts cleanly and evenly; the rosin yellow and pure white crumble very easily; the lotus green (a variegated white/green) breaks off in chunks. I've learned that I can make small cuts with a tile nipper, which helps too.
I have learned something about marble chemistry. When I found out that blue marble is (gasp!) $70-$80 per square foot, I adjusted the design so I'd need only a third of what I originally planned, because I certainly didn't budget for that! A friend in the tile industry told me that that's because blue is not only scarce, but that what makes the blue color in marble is gold. Who knew?
This method also has its particular hazards. While it doesn't have the sharp edges that glass does, every time I whack that hammer I run the risk of also whacking fingers and thumbs, which I've done more times than I like to think about. Somebody suggested holding the piece I'm whacking with long tweezers - brilliant! The only problem is holding the tweezers steady enough.
But progress is being made. I've almost finished the two orchid pots that are the main features of this design; most of the rest is background, which will be a breeze.