We don't have synonyms for hearthstone yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are heartstone, earth tone and hearthstones.
(n) :
a flat stone used to form a hearth; by extension: The fireside, homelife
(v) :
to scour, as a floor, with hearthstone
"1876: Hallberger's Illustrated Magazine, page 202 - We've a woman come in twice a week, to scrub, and red-brick, and hearthstone, and black-lead, and the rest we manage ourselves."
(n) :
a flat stone used to form a hearth; by extension: The fireside, homelife
(v) :
to scour, as a floor, with hearthstone
"1876: Hallberger's Illustrated Magazine, page 202 - We've a woman come in twice a week, to scrub, and red-brick, and hearthstone, and black-lead, and the rest we manage ourselves."