I hand painted a seamless cathedral floor texture. It's loosely based upon this reference, but mostly my own design. Perfectly tileable.
First I had to put together a base texture. I came up with this. It has a kind of sandstone feel to which is basically what I was going for.
Then I got to the line art and detailing the shape of each stone. I wanted to make it appear worn and cracked. I added some color variation to spice it up a bit along with the dings and divots in the surface.
These should both be fully seamless but let me know if you think you spot any and I'll see if I can correct it.