The most common and most likely answer to this crossword clue is the 5 letter word COPAL
I've seen this crossword clue in The Guardian
C
O
P
A
L
More clues leading to the result COPAL
Buddy explains!
The straight part
We know that "hard material" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hard material = copal.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 COPAL mean?
COPAL noun- A resinous exudation from various tropical trees, especially Hymenaea courbaril and Schinus terebinthifolia, used chiefly in making varnishes and printing ink.