This is Clara's signature Season 7 necklace. It was sold out long before it was ID'd, so the real one is extremely rare. However, alts are easy to find. There is a silver/blue one by Forever21 that many people buy and paint the stone black with nail polish. Some also paint the rest gold, others just leave it silver. There is also a gold/black one by Forever21, but that version is almost as rare as the SA necklace.
Worn in: The Bells of St John (outfit #1), The Bells of St. John (outfit #2), The Rings of Akhaten, Journey to the Center of the TARDIS (outfit #2),The Crimson Horror (outfit #3).
Urban Outfitters Verameat Owl Necklace in Gold
I'm 85% sure of this ID, the only difference that bothers me is that on Clara's the chain is attached to a gold loop, and on this one the chain rests behind the pendant. Urban Outfitters could easily have turned the loop down for the photo, we've seen that before, but until someone gets ahold of one and confirms one way or the other, I think this is the most likely ID.
Worn in: Hide
NW3 Hobbs Dormouse Pendant
This is very rare, and unique enough to be hard to find an alt for. I'm still waiting for the day that someone who does have the necklace will make a mold and sell replicas. There is a little horseshoe on the extension chain that is twisted around to the front when Clara wears it.
Worn in: The Name of the Doctor
NW3 Hobbs Coral Horseshoe Necklace
This necklace is not entirely rare, you just have to be willing to pay a lot to get one usually.
Worn in: The Day of the Doctor
Shoptiques Three-Cone Necklace by Double-Dutch Boutique
This necklace was sold by a small artist shop called Double-Dutch Boutique on the website Shoptiques (kind of like Etsy). Once the owner was informed of the necklace's importance to a lot of cosplayers, they were restocked for a while. They aren't currently in stock, but it's worth checking every once in a while if you still want one. Also, since they were individually made, there are sometimes slight differences between some SA necklaces; length, extension chain, type of clasp, etc.
Urban Outfitters Double Circle Pendant Necklace
Very easy to find or make alts for.
Worn in: Into the Dalek (outfit #1), Into the Dalek (outfit #2)
Urban Outfitters Tiny Triangle Necklace in Gold
On older necklace by Urban Outfitters, and was ID'd later. This is one that cosplayers will have to keep a watch for on eBay and Vinted and such until a few are found.
Worn in: The Caretaker
Topshop Circle Pendant or Pamela Love Arch Pendant Necklace
We've seen several versions of this necklace. There was one by Barney's for over $6,000, there was one by Pamela Love for over $275, and there was an older one by Topshop. I tend to believe it is the Topshop necklace, just because I can't see them spending that much on a necklace, especially when the rest of the outfit was already moderately expensive. Plus, 95% of Clara's jewelry has been Topshop or Urban Outfitters.