The most common and most likely answer to this crossword clue is the 9 letter word AMETHYSTS
A
M
E
T
H
Y
S
T
S
More clues leading to the result AMETHYSTS
Buddy explains!

The straight part
We know that "gemstones" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gemstones = amethysts.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 AMETHYSTS mean?
AMETHYSTS noun- A transparent purple variety of quartz, used as a gemstone.
- A purple colour.
- The purple tincture when emblazoning the arms of the English nobility.