Belemnites are an extinct order of cephalopods (“cephalo” meaning head and “pod” meaning leg) that lived during the Mesozoic era, some 200 to 65 million years ago. They were elongated organisms, resembling today’s squids, only tinier and cuter.
They were prey animals and had ten hooked tentacles that they used to catch and hold their food.
What sets them apart from squids is their internal skeleton, bullet-like structures called guards. The dazzling display you see here is one such guard that fossilized through a process known as opalization, by which organic matter is substituted with amorphous silica. In layman terms, this means replacing tissue with a non-crystallized, hydrated form of silicon dioxide.
This amorphous (glass-like) property causes opal to diffract light differently throughout its mass, giving the stone its hue and flecks of color as it catches rays of light.