There is no known stone called "hial stone." It's possible you might be thinking of a similar-sounding term, or it could be a misspelling.

Could you please provide more context or details about what you're looking for? Perhaps you could:

* Double-check the spelling.

* Tell me where you encountered this term.

* Provide any additional information you have about it.

This will help me understand your request and provide you with a more accurate answer.