This one is made in a factory. The normal is beige, the company also makes white, blue, and black. The beige is the only one not colored.
The only naturally black beaches I have seen had gravel, not sand. That was in Hawaii.
No, silica based sands will not help with Ca
, etc. I do have oolite in my fuge which is the best at going in and out of solution to help with those things though. Then there is always water changes, supplementation, etc.
Those bottom caves are not as dark in real life as that pic shows, but I am sure are darker than if the sand was white. I prefer it this way, I like scallops, sponges, and other filter feeders that like their darkness.
Honestly I don't think black is any less natural. The natural reefs I have seen (Hawaii and Virgin Islands) were not the bright white like most aquarium sands.
Yes, there was an article in a fish magazine a little while ago using a light meter in a tank with a dark blue background that showed how the dark background measurably reduced the amount of light. But it is not like dark backgrounds mean corals near them suffer at all, at least not that I have ever seen.
Again, I prefer bright light with a dark background to increase coloration and show it off better. I read somewhere that dark backgrounds and substrates actually increase the actual amount of pigmentation produced by fish, they are physiologically more colorful.