Where does “Ratna” come from?

Ratna derives from Sanskrit रत्न, which is cognate with Sanskrit राति, though the semantic connection between these forms remains unclear.

Ancestry of “Ratna”, step by step

Step	Language	Word	Meaning
1	Sanskrit	रत्न	gift, present, goods, wealth, riches; jewel, gem,...
2	Sanskrit	राति	a favor, grace, gift, oblation; "the Giver";...

Ratna — Etymology, Origin & Word History | Etymology Explorer