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I know that you are very knowledgeable in geology/mineralogy, and my knowledge is limited. On my previous post, I included both types, as a general definition of the word "crystal".
But in that...

The crystallization process takes a lot of time as I'm sure you know, and, although, you are right, about every mineral having a gloss surface, most of their crystals are not visible on naked eye....

Now you're making me search for old articles, take a look at this An Object Lesson in Japanese Hones: Part II | Eastern Smooth: The Blog , and, read from Jim's blog, it's extremely interesting. But...

There is an old rule for Jnats; if the inclusion shines-reflects light, then it's toxic. And I'm not referring to the shining ones that become this way after honing, from polishing, those are softer...

I had the impression that quartz does not appear in Jnats. But if it's white or transparent, I guess it could be one. And like all the hard inclusions, it needs to be removed.
Use something hard...