Based on the end of IM3 (the necklace he gives to pepper) Tony is the sentimental sap who proposes by taking the pieces of shrapnel from his heart and melting them down or something to use in the ring he proposes to Steve with. Maybe Steve had carried his ma's ring around with him before the serum, and so he has it with him in the ice and that's what he proposes to Tony with.
OH MY GOD THAT'S SO ROMANTIC?? tony giving steve a piece of something that pierced his heart because *steve* pierces his heart and steve giving tony the only thing he has of his mother's because he's trusting his future to keep his past safe ;_;















