Ammonites

Blue Cleoniceras

Elsewhere on the site, you'll find polished versions of these, which are amazing in their own right. In some examples, though, the iridescent layer is so vividly coloured that it'd be a shame to polish through it - so they don't. 

They are mostly Cleoniceras cleon, but all are 110 million years old, and from Majunga in Madagascar.

4-5cm