For those who didn't know, this is an opalised Belemnite fossil 

from the Jurassic period, found in Coober Pedy, South Australia. 

Similar to the modern squid, the Belemnite is an extinct marine cephalopod. From its aragonite phragmacone would emerge arms equipped with small hooks for grabbing prey passing in the prehistoric waters.

Somebody - not me, of course, just somebody 

might notice it looks like a unicorne horn. Just saying.

(Source: malformalady.tumblr.com)