Anne, a 'cabochon' is typically flat (often rough) on one side and finished on the other. A cab can be smooth, textured, or carved. Modern carving tools make it possible to carve many types of stone. The scarab is totally normal in that respect.

My guess is one or more of the original stones has been replaced.

My suggestion of amazonite (the Russian variety) is only an educated guess based on appearance in the photo. Only you can see the finer details, like the crystalline structure of stone, or the bubbles and swirls of glass. Glass? I don't see that here, but I could be mistaken.

I agree about the entire necklace being very attractive. VERY beautiful.