The problem is related to the definition of "supernatural" as referring to ontologically basic mental things.

Not everyone who believes that a stone is healing power believes that they are ontologically basic.

A stone that does "healing" would be like having a type of acid that only dissolves shirts--it has no way to know whether something is helpful or harmful any more than the acid has a way to know that something is a shirt.

If you have an ill person telling them to get a good nights sleep, helps them heal in a fairly diverse set of circumstances. The advice isn't helpful in every case.

Frankly, any stone that was powerful enough to "heal" is something I wouldn't trust since pretty much any singificant "healing" effect could cause really bad harm under the wrong circumstances.

The question whether or not you trust the stone is irrelevant to the question of what's a useful way to check to CronoDAS girlfriend.

In practice she might tell you: "Duh, of course I check with a trustworthy spirit whether the stone is right for the particular occasion."

A quick googling for hematite suggests that it's supposed to grounding and balancing energy. Given that the girl is ungrounded to the extend that she sees spirits, from her perspective getting a stone to ground herself makes a lot of sense.