I would think that the characteristics if a good finishing stone would be one that has a composition of very fine grained(if not microcrystaline) material which is harder than steel, but with a homogenous presence throughout the stone.

I cannot answer to the question about the a
"active" cutting material in a Japanese water stone. Interesting to look into. I do not own one. But will soon enough.

Porphyroblasts only refer to the minerals in a Schist or other metamorphic rocks which is not the prodominant mineral. In your case the prodominant material would be chlorite. If you prove the presence of porphyroblasts which are harder than steel(through the scratch test), but are not present to the naked eye, the only way to identify them is under a polarizing microscope.

This can get very technical, but what is important is grain size and if the material is harder than steel.