He's my sun, he makes me shine like diamonds
The reflection of light is what makes the diamonds so beautiful and clear.
Just like the sun with diamonds, the one she loves makes her shine.
Diamonds sparkle and given the right amount of light can almost blind you like the sun. Here she alludes to his beauty as well as hers and how deeply in love she is with Gatsby.
Assuming that Lana is singing in the voice of Daisy Buchanen, much of Daisy seems ethereal, divine and rich. The idea that Nick narrates Daisy’s beauty creates the impression that she shines. When coupled with Gatsby’s intense fixation on Daisy that eventually draws Tom’s attention, it is almost like Gatsby’s love for her causes her to sparkle.
This plays perfectly with the fact that much of Daisy’s beauty is in fact built on Gatsby’s idealogical concept of her. Without which Daisy would just be an ageing rich girl with a cheating husband.
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