I apologize for the above picture. The green gem is much bolder in person. This is the official Sailor Jupiter tiara from GE Animation.
The gem is a light green. The package says it's for ages 15 and up, so please do not think this is a kids' toy. There are thick sheets of white plastic on the sides of the tiara to help curve the tiara over your head. There's an elastic band in the back, but it fits tight and snug over my head.
To put it on, slip the tiara over your head, flip your hair over it, then slide the tiara over your forehead with the elastic band high on the back of your head. Like a headband for your face. If you have a Sailor Jupiter wig, clip your hair up and slip the wig on, using the bangs to cover the plastic.
In my case, I do not yet have a Jupiter wig. So I'll just cosplay as myself, Codename Sailor Earth.
Styling my curls with my fingers, I covered up the plastic sides pretty well. Now I just need a new costume so I can cosplay as my own Guardian!
I already found my earrings!