"Jus' a piece a slack lime. Foun' a big hunk."
Slaked lime, or calcium hydroxide, is a crystal or white powder usually found around volcanic, plutonic and metamorphic rocks.  It's relatively rare, but has sometimes been found in burning coal dumps.  Ingesting the chemical can lead to vomiting, diarrhea and severe pain, although these effects might not have been widely known at the time.