The ruby stone is a solid high abrasive density, but very hard fine alumina stone from China. The advertised grit rating is often 3000, and at least as delivered, the stone is probably in the ballpark of a 4000 grit waterstone, but with abrasive particles fixed.
Expect over time, the stones will become finer and slower, and if you don’t favor that, you may need to use a fine diamond hone once in a while.
The ruby color is the abrasive itself (even though it might be slightly different than ruby) and I find initially, the stone works great following stones that are too coarse for a finer stone to follow. And with the buffer doing a light buff strop at the end of the process, the edge is very fine, without the same rounding that you get from slurry dulling on a fast cutting waterstone.
Too, when shopping for a stone of this type (This one came from aliexpress), if you find a bunch of $50 with shipping stones and you’re feeling frugal, you can look for the combination stones. Half will be ruby, half will be something else. if you intend to use the ruby as a single stone, you can ignore what’s on the other half of the thickness because in this case, you’ll never in five lifetimes work through half an inch of ceramic-like stone. Far better to have a 1 inch thick combo stone than to buy a 1/2″ thick stone only to save money. You’ll appreciate the height.
Initially, it’s subjectively almost as hard as a spyderco stone. time will tell if it settles in to the same fineness. Best followed by light buffing or a strop with fine compound on it with a moderate touch.
The capability of aluminum oxide doesn’t change much from one type to another, but more with how it’s affixed in a stone. That means this stone will sharpen anything from a cold chisel to PM V11, but it will not deal with vanadium carbides properly without sprinkling diamond powder on the surface.
Of Making by Hand with Wood and Steel 