when i first saw it i thought it was a plumb bob. but now i'm not so sure. i've had it at least 25 years but i never used it or am willing to toss it because some day it might come in handy to beat into a hole in something to make it bigger
thats a carbide inset from either an asphault grinder or a trencher
EXTREMELY hard!
grind it down to a fine point and it would make a great center punch
I've changed thousands of those on big augers, "hole saw" shaped drills, etc. I remember on one job we hit rock that was insanely hard. 30 -60 seconds of drilling and the teeth were gone. I hated when they got so hot that they wouldn't come out or when ran to long you couldn't get the tool in the groove to hammer them out.