Walnut is mentioned as a potential source of natural pigment for coloring hypertufa.

This got me to wondering whether ground walnut shell might be an acceptable sand substitute for hypertufa. It's fairly lightweight compared to sand, is a nice tan color and seems to be quite uniform in size.

I see this stuff sold at Petsmart and other pet stores for use as a reptile substrate.

Although it's more course than walnut shell, turface/profile is about the same color, and is probably even lighter. For the uninitiated, this is a baked clay product that is used in landscaping as a soil ammendment.

I use it (sold as Schulz aquatic plant soil) as a substrate in some of my aquariums.

Does anyone have experience with either of these as aggregate in a hypertufa mix?