The most common and most likely answer to this crossword clue is the 6 letter word SPINEL
I've seen this crossword clue in Financial Times
S
P
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More clues leading to the result SPINEL
Buddy explains!
The straight part
We know that "Mineral crystal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mineral crystal = spinel.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 SPINEL mean?
SPINEL noun- Any of several hard minerals of cubic symmetry that are mixed oxides of magnesium and aluminium and are used as gemstones of various colours.
- Any crystalline material, not necessarily an oxide, that possesses the same crystal structure as this mineral.
- Bleached yarn in making the linen tape called inkle; unwrought inkle.