How can magnatite be distinguished from black spinel?
Black, opaque stones can be tough… For magnetite, one approach might be using its magnetic properties: If you have a piece of Styrofoam, make a small float that you can put in a sink full of water. On the float put something like a nail or paperclip… that is attracted to magnets. The magnetite should create enough of a field to cause the iron to be pulled (float) gently toward the magnetite.
While black spinel is magnetically susceptible, it does not have its own magnetic field. Have a read here too: