The most common and most likely answer to this crossword clue is the 9 letter word SAPPHIRES
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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 = sapphires.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 SAPPHIRES mean?
SAPPHIRES noun- A clear deep blue variety of corundum, valued as a precious stone.
- A white, yellow, or purple variety of corundum, either clear or translucent.
- A deep blue colour.
- Any hummingbird in the genera Hylocharis and Chlorestes, as well as the rufous-throated sapphire, which is now in the genus Amazilia.
- Any of the butterflies in the southern Asian lycaenid genus Heliophorus.