The most common and most likely answer to this crossword clue is the 6 letter word PUMICE
I've seen this crossword clue in The Guardian
P
U
M
I
C
E
More clues leading to the result PUMICE
- This person's about to enter a shade abrasive
- See rodents after turning up stone
- Abrasive rocks cropped wild cat earlier
- I’m standing in purplish brown lava, turned to stone
- Volcanic rock turned up by little creatures
- Smoother on a horse, contrary timid creatures
- Stone upturned by vermin
- Turned up with peripherals – rough stuff
- Half pound hail stone
- Polish unfinished shoe with hard substance
Buddy explains!

The straight part
We know that "Volcanic rock" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case volcanic rock = pumice.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PUMICE mean?
PUMICE noun- A light, porous type of pyroclastic igneous rock, formed during explosive volcanic eruptions when liquid lava is ejected into water or air as a froth containing masses of gas bubbles. As the lava solidifies, the bubbles are frozen into the rock.
- To abrade or roughen with pumice.