The most common and most likely answer to this crossword clue is the 9 letter word RUBELLITE
R
U
B
E
L
L
I
T
E
Buddy explains!
The straight part
We know that "Red stone" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case red stone = rubellite.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 RUBELLITE mean?
RUBELLITE noun- A red to violet variety of tourmaline used as a gemstone.