I had to look it up also. :) So it seems to be a single precious stone pendant on a necklace. If you click on the russian-language page of Arucard's link, you'll see a photo. Turns out it's also the name for a lapel mic, but I doubt one would be made of diamond.

lavaliere: an ornament hanging from a chain, worn around the neck A pendant worn on a chain around the neck. http://www.yourdictionary.com/lavaliere

A type of jewelery http://en.m.wikipedia.org/wiki/Lavalier?wasRedirected=true

is it expensive?

i don't know,,,,