flourite

The two will mix together in a matter of months. Save yourself the time and just mix a bag of Flourite with, in your case another 10 - 15lbs. of natural plain old aquarium gravel. Rinse them and put down a light dusting of ground peat moss first an lay the mixed substrates in.
I did this in one of my tanks and it looks nice and works well. I used about a 60/40 mix of Flourite/natural gravel. It just lightens up the color a bit.

Len

But what is the point if you are not having live plants? I would not bother.

My substrates are quite different in FO in planted. The biggest difference is the amount of substrate - FO tanks have the minimum substrate to cover the bottom glass (depends on tank size and the fish involved, but ~1/2 - 1 1.2" max, with the deep ones being RFUG anyway) so that it can be kept clean. Planted tanks have 3-6". To shift from one to the other I reset with a different substrate. Most of the fish in my unplanted tanks are not suited to plants in any case (herbivores, diggers, etc.). Having a deep substrate in an unplanted tank is for me an upkeep hassle I don't want or need.