Dragons Hearts/Eggs are forms of Septarian Nodules (sourced from Madagascar). Over millions of years - the nodules slowly form with fossilized mud bubbles helping it take its form as a result of volcanic eruptions- slowly encasing decomposing minerals and dead sea life. Composed with volcanic ash, calcite, aragonite while coated in limestone.
This is a stunning piece.  Polished back shaped into a subtle heart. Gives off the most grounding energy for space and is an incredible collectors piece.