I only just found this thread, and haven't had the time to read all previous posts, but here is my experience with 3M Color Quartz.
I found Tahitian Moon Sand outrageously expensive, and since the Black Beauty is essentially a waste product, you just don't know what you are getting. Maybe my background in chemistry makes me over-careful, but I am not convinced that some batch of it couldn't leach toxins into the water. Even a trace of heavy metal salts could kill off your fish in no time.
I found the Color Quartz to be a perfect solution - less than half the cost of Tahitian Moon Sand, and a carefully engineered product that's guaranteed to be chemically inert. Not even regular sand offers the latter!
Following the advice of others on this forum, I rang a raft of pool and tile shops in my area, but none of them had ever heard of 3M Color Quartz. I ended up calling 3M at 1-800-447-2914 (the phone number is on their web site burried in the FAQ about Color Quartz at http://authoring.3m.com/cms/US/en/2-125 ... view.jhtml
). I was very lucky in that the 3M distributor of Color Quartz for 5 States happens to be located only about 30min drive from my home, and I bought the Color Quartz directly from them. Unfortunately they charged $22 for a 50 pound bag of black T-grade Color Quartz, which is slighly more than people have paid elsewhere in the US, but I still found it well worth the money.
For those of you in or around NE Ohio, the distributor is N.T. Ruddock Company, 26123 Broadway Avenue, Cleveland, Ohio 44146, Tel: (440) 439-4976, Fax: (440) 439-8728). Either 3M (anywhere in the US) or this distributor (around NE Ohio) should be able to point you towards a store where you can buy the stuff.
When I bought it, I also saw the S-grade Color Quartz, which is finer grained and has rounder granules. The T-grade looks and behaves like sand, I definetely would not call it gravel, but it is a comparatively coarse sand - not sharp though. The S-grade is very fine, more like a powder, and feels very very smooth, due to the round particles. For pool applications, I believe they use S-grade if you want a very smooth surface. The T-grade is used where you want a bit of grip; eg on stairs. Both would probably work well as substrate, but I thought the T grade would look more natural - if that's possible with a black substrate.
The 3M distributor where I bought the stuff had a sample box with little test tubes full of different types of Color Quartz in all the different colors. They also had a similar product for roofing applications that was just a notch coarser than T-grade. I forgot the name, but it would probably act like a very very fine gravel. You might want to try and find a store in your area that has the stuff and check it out yourself. That's the only way you can find out which of the different options you prefer.