My rendition of the necklace Yuki Cross wears. I know there's a premade one, but it wasn't accurate enough for my liking (I own it, and it's nothing more than the Academy symbol on a black string), so I made my own. The charm was made using Sculpey, which I painted using silver craft paint. Unfortunately, regardless of what I tried, the craft paint wouldn't match the beads, so there is a slight difference in tone. Fortunately, this can't really be seen unless you see the necklace up close, so it didn't matter too much. :).
Anyway, the next part was the actual necklace itself. For this, I bought some beads from Michael's and Hobby Lobby that I felt worked perfectly for this accessory. The diamond beads are from Michael's, and cost about $5-$6 for a strand of three. This SHOULD be enough for your necklace, but if you're not sure, buy a second strand to be safe.
The circle beads came in a baggy, and cost about $5 for the whole thing. They were made of nickel or steel (I don't remember which), so keep this in mind if you're sensitive or allergic to these materials.
As for amount, one bag will DEFINITELY be enough.
I then assembled it, and the result is the necklace.