Two thoughts:

One is that irregular slate flagstone is often used for indoor floors. If can be really rustic, like this:

A second thought is that there are lots of gorgeous non-square tiles, like these:
{{gwi:1445515}}

Do you want gray specifically? Home Depot sells some stone look porcelain that looks like a random cut stone floor from Merola in the Elba line.

stacyneil and brickton! There ya go. I knew I was smart to begin my search on here. I like the idea of bringing the garden inside and now see I have that option...thanks so much for all the info and pics...greatly appreciated! Now I will begin my research IRL :>)

We have a black slate floor in our kitchen and I love the look. We live on top of a mountain in the forest so we tend to stick with more natural materials. At night the slate almost reminds me of a black leather floor. It isn't slippery and, while it does require occasional sealer, it is very forgiving.
http://www.digsdigs.com/nkba-2008-award-large-bathrooms/
The NKBA '08 1st place bathroom (1st pic in this link) has a floor similar to what you might be interested in. This bathroom is part of my inspiration and we are considering this type of tile. I think it might be slate. It comes in sheets already broken in random patterns or, you can buy the squares and have a tile professional break it and install it.