The most common and most likely answer to this crossword clue is the 6 letter word BERYLS
B
E
R
Y
L
S
Buddy explains!

The straight part
We know that "Gems" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gems = beryls.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 BERYLS mean?
BERYLS noun- A mineral of pegmatite deposits, often used as a gemstone.
- An example (a stone) of the mineral beryl.
- A dull blueish green colour.