Is stony yet may be worth millions! (3)
I believe the answer is:
gem
'be worth millions' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for gem that I've seen before include "36 Across or any other jewel" , "Setter might bed this" , "A jewel of an Orangeman" , "Valuable stone" , "Pearl" .)