The most common and most likely answer to this crossword clue is the 6 letter word ZIRCON
I've seen this crossword clue in The Guardian
Z
I
R
C
O
N
Buddy explains!
The straight part
We know that "mineral in crystal form" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mineral in crystal form = zircon.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 ZIRCON mean?
ZIRCON noun- A mineral occurring in tetragonal crystals, usually of a brown or grey colour and consisting of silica and zirconia.
- A crystal of zircon, sometimes used as a false gemstone.