Deep red gemstone (6)
I believe the answer is:
garnet
'deep red gemstone' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for garnet that I've seen before include "Glassy red, yellow or green gemstone" , "Precious stone, usually red" , "Semi-precious stone of deep transparent red" , "Deep red gem stone" , "January birthstone" .)