I think I see what you mean,
but that chipping is something you'd also see on natural amber (layer cast resin

)
shells almost always have an iridescent inner layer ("mother of perl" - esque so to say)
it's still quite possibly a shell (I can't tell from the pics to be honest)
But I'd not stick with shells searching for a similar pendant..
horn doesn't chip that way (it's more like hair tearing in fibres)
so that I'd indeed rule out.
but amber would chip, so would a gemstone
black Jade (should still have some form of greenish tint)
some form of Agate (Onyx for example is mostly black but can brown as well)
would chip (with extremely sharp edges)
Onyx blades are still the sharpest blades possible (and the most brittle ones)
and what's more important is likely the most carved gemstone

or dark Carnelian... (Chalcedony)
Anyways, I wouldn't stick with just shells if I search for a similar pendant on the internets,
even if it is, the seller might mistook it for something else.
personally seeing that back images.. I'd even tend to say it'll be Agate
see one agate flower for comparison
https://www.etsy.com/listing/457334484/b725-a-lovely-vintage-925-sterling
and a carnelian:
http://www.saharscreations.com/Carn...lace-with-Carnelian-Flower-Pendant-p2007.html
'sid