Did you mayhaps mean hearthstone?
Definitions
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Examples
“He caught at the heartstone to catch himself; but the heartstone was stronger than Dwvon, and his fingers could not pierce it.”
“Inach is heartstone in the human tongue," Nolo answered.”
“The trip to Malmoret would still be planned, and the swords the Dwarves would fashion from the heartstone in the henges would need strong arms to wield them.”
“The only thing he didn't mention was that the henge was made of heartstone.”
“He could neither descend through the fountain nor tunnel through the heartstone around it.”
“But you must have found pieces of heartstone you could cut with before," said Redburr.”
“Slowly he worked his way deeper into the rock until at last he came to a place where the heartstone itself was riven.”
“There's enough heartstone here to carve into a thousand swords.”
“Blots of darker heartstone marked other islands amid the tongues of yellow flame.”
“Then Dwvon realized he lay atop another plug of nirrin in the heartstone.”
Lists
These user-created lists contain the word ‘heartstone’.
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Substancestry
Mysterious and theoretical substances and "stuff" of legend. More emphasis on the ancient, mystical, mythical, folklore, mathematical, and scientific. I won't be listing too many "sci-fi" or comed...
ylem, ichor, aether, ectoplasm, impossible object, quark star, eucharist, pixie dust, eitr, elixir of life, philosopher's stone, alkahest and 126 more...
Tweets
Looking for tweets for heartstone.

yarb Seanahan's comment made me laugh so much at the time I was incapacitated, and therefore could not comment; finally, after four months I am able to do so. Oct 2, 2008
seanahan "This has never been proved definitively"? Really, nobody has ever proved that? May 27, 2008
whichbe A fictional artifact common in dragon-related stories of high fantasy. It is a crystal of concentrated magic, and can be extracted from the heart of a slain dragon. It is believed that the heartstone is the source of a dragon's magic power, but this has never been proven definitively. Heartstones were considered a precious commodity in the Middle Ages, and were coveted for their wizard killing abilities. Generally the magnitude of a heartstone is directly related to the size of the dragon itself, though this is not always the case. (Wikipedia) May 22, 2008