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He's letting her touch his Gem.
Why is this such a big show of trust? Because Steven has never let anyone else do this. His Gem is the source of his powers and his connection to his mother/Gem Heritage.
Allowing someone else to touch it is like allowing half of what makes him who he is to be in the palm of another person.
And after the events of CYM where White Diamond did what she did to him, it says a lot about how much trust and love Steven has for Connie if he lets her do something that he never even lets the Gems do.
