It is similar to both but I would call it a banded opaque.
Of course every body that looks at it will say it looks like a clam or a Indian, But it's a banded opaque in my book.
Unusual for sure, great find.
Randy
no·men·cla·ture
/ˈnōmənˌklāCHər/
noun
noun: nomenclature
the devising or choosing of names for things, especially in a science or other discipline.
•the body or system of names in a particular field.
plural noun: nomenclatures
"the nomenclature of chemical compounds"
•formal
the term or terms applied to someone or something.
"“customers” was preferred to the original nomenclature “passengers.”"
THERE This may help LOL
I know all things.
Just kidding but I spend time on coin collecting boards they seem to be a lot more active and I learn a lot there like how to tell how much a excepted best offer was.
Here is a web site that has the info. You need to enter the eBay item number
Some time I can not get it to show but normally I can.
http://www.watchcount.com/
Enter the item number where it says key words
Randy
Original Box Of Antique Christensen Agate Guinea Marbles Estate Fresh!
See original listing
Item condition: Used
Ended: 16 Dec, 201422:48:12 GMT
Sold for: US $3000.00