Featured Old necklace 10k? Help to identify the stone?

Bough this today, I quite like it for myself. However, I am completely stumped on it. The marking on the clasp appears to say 10k but then an E? Maybe a makers mark? Also, the stone as always I have no idea. The tag said it was Mastodon but I still don't know. The part holding the stone is definitely gold and appears to be quite old? Any help is appreciated!

 

the materiel looks natural , like some bone or horn...

 

Looks like a tooth.

 

Root beer float stone?

 

An E after a karatage mark always makes me think 'electroplate'.

The pendant photographs more like shell. Does it feel like stone in the hand? No real reason it couldn't be a fragment of mastodon tooth or bone. Teeth:

 

It could be fossil oosik also.

 

I don't think that chain is electroplated.... clasp dates it to 20s-30s? Probably a hallmark.

 

That loop on the tooth/whatever looks plated, but the chain doesn't. Go figure.

 

Would it make sense to use a harder metal wire there with a heavier piece of using this type of construction?

 

If that's as old as it looks, you never know. The old school jewelry types did all sorts of weird stuff.

 

The mark may apply only to the clasp. It is that older, life preserver, style of spring ring.

 

The pendant is zebu horn, very popular in the 19th century, when natural materials were en vogue. The style is 19th-early 20th century.
The cap of the mount looks rose gold. The chain could be original or a later addition. As Bronwen said, it is an old style spring clasp, 19th century. Spring clasps were invented in the first half of the 19th century.

Zebu horn:


Zebu, this one has the colour scheme of your pendant:

 

Thank you guys! Only paid $30 for it and I plan on wearing it as it looks quite nice.

 

The piece turned out to be 10k. I just need a chain extender and I can wear it

 

Thanks for the chuckle!

 

Looks to me like a claw/bone combo that's been polished down; from what, who knows?