That little glimpse of light
Makes that diamond really shine
And you already is a star

from Beyoncé (Ft. Jay Z) – Upgrade You Lyrics on Rap Genius

Meaning

She doesn’t need a lot of attention (‘little glimpse of light’), to look good.
‘Makes that diamond really shine’, diamond could represent just in general all of the new thing’s she’s gained from being with him (probably one of those things being a diamond), and how it’s noticeable by anyone.
But she was already a star.

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