Burnish Stone

"A coarse stone that's chock full of holes." -Description

Obtaining
Burnish Stones can be obtained in numerous ways:
 * Chests in dungeons
 * Crates washed-up on the shoreline opened by Arfur
 * Quest rewards
 * Sold by Snorkel, the cat merchant, for roughly 300 celestial shards (not always available)
 * Sold by City Mayor (A yellow bird), for antidote (not always available)

Use
It is used, along with Iron Nugget, Moebius Coal and Nova Stone, to upgrade the Hero’s Sword. Talk to the Blacksmith to do so.