Our friend has the most beautiful slate floors in the kitchen, dining room, and living room of her Eichler. It turns out it's from Echeguren Slate, Inc. in San Francisco. She's used 16x16 squares of China Black, which cost about $6 per square foot. The floors really show off good rugs, and they also look great naked. They're not cold -- even when the Eichler's classic radiant heating is off -- and they're also durable and easy to clean. (However, you have to be sure to use the right sealant.) The slate can be used either inside or out. You can see a bit of the slate in these photos of a house that our friend (a designer) worked on:
The website says that the China Black slate is medium to hard in density and has an average cleft.