Killer modeling! I agree that the texture could use a bit more variation. It's also missing that "micro specularity" you get with sandstone - that little sparkle you see from light hitting the quartz chrystal grannules. Not sure how you could accomplish that without killing the render time though........
Great work!
"Skill without imagination is craftsmanship and gives us many useful objects such as wickerwork picnic baskets. Imagination without skill gives us modern art." - Tom Stoppard