How do you tell between three different stones when they look so similar? Gemstones are classified based on their chemical and crystal structures which give them defining characteristics like color, hardness, luster, and density. Most of us do not have specialized machinery to analyze similar gemstones on the spot, so we need a way to quickly separate samples by eye or by feel.
A good start is to feel the relative weights/ densities of the stones. All three of these samples are roughly the same size and look very similar in color. We can quickly tell that Spinel is much denser than tourmaline and agate when we hold it in our hand.
Tourmaline is only slightly more dense than Onyx, so it would be difficult to tell between the two just by feel. Adding to the difficulty is that they both register 6.5-7.5 on the Mohs Hardness Scale. They are obviously both black and opaque, so is this game over for us? Not at all.
Tourmaline has a much better luster when cut and polished than Onyx. Though black in color, it reflects light more brilliantly than the Agate family which Onyx is a part of.
To sum things up:
Though you would need gemological background to know the names of unknown gemstones, there are many different easy tricks we can use to distinguish between unknown gemstones.
-Cyrus Nemani, Gempacked