I believe that what you have is a type of stone called basalt. It is the most common composition of lava rock. I use an app called rock identifier that is really useful, highly suggested!

Sweet find!! Fun fact, the positive side would actually be a petrifaction or an organism that’s been replaced by mineral or stone and the negative is a true fossilization or an imprint of an organism.

These look like coral polyps! The top three look like cross sections where as the bottom piece you can see more of a vertical shaping.