Green’s Dictionary of Slang

hot rocks n.2

(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.

[Ire]G. Coughlan Everyday Eng. and Sl. [Internet] Hot Rocks (n): The burning bits of hash/paper that flake off from the business end of a joint.
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.
[UK]Guardian On Line 24 Aug. [Internet] Hang on, there’s something in my eye. Although it might be just a rogue hot rock .