These are natural opal fossils from a belemnite. They can be polished/carved for unusual jewelery or keep as specimens.
The area of what is now Coober Pedy, Australia, was once covered app 100 million years ago by a large saltwater sea. There was abundant sea life in it and many of the opal deposits in the area are the hiding place of fossil opal that is shaped by the creatures from the area such as seashells and this fossil squid like creature called a belemnite.
We have taken the photo with the rough wet.