Ruby, it seems, softens when warmed up (3)
I believe the answer is:
wax
This clue could be a double definition.
'ruby' is the first definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'it seems softens when warmed up' is the second definition.
The definition suggests a singular noun which matches the answer.
Can you help me to learn more?
(Other definitions for wax that I've seen before include "Polish" , "Increase in strength or size, like ear matter" , "Yellow stuff secreted in ear" , "increase like stuff in ears" , "become stronger" .)