Answer:
If you have me, you want to tell me. If you tell me, you don't have me.
It's what light turns into in the night
Glittering points that downward thrust. Sparkling spears that do not rust. What is it?
I can be as thin as a picture frame but my insides have many things you can see.
Almost everyone needs it, asks for it, gives it. But almost nobody takes it.
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?