Green’s Dictionary of Slang

(Irish/Aus. drugs) hot ash, dropped from a joint, which makes small burn holes; also, when sucked through a pipe and it burns one’s throat.

2001		G. Coughlan Everyday Eng. and Sl. 🌐 Hot Rocks (n): The burning bits of hash/paper that flake off from the business end of a joint.
2003		beautifullytwisted.co.uk 26 Nov. [blog] She’s dropped a sprog – who I’m worried for, since there were hotrock holes all over her couch.
2013		Guardian On Line 24 Aug. 🌐 Hang on, there’s something in my eye. Although it might be just a rogue hot rock .