Glue on chain loop.  Has a dark blue pad.
 
Can diffuse essential oils or perfume by dropping it in the center of the pad.  Use as a necklace with an 18" chain and you can smell the scent.  If you are using it as a diffuser and want to use it on a bracelet or longer necklace , you will have to bring the jewelry base up to your nose to smell it.
 
Diffuser is 1 1/4" in diameter.