Time in your hands
The Nerinea is an extinct genus of sea snails. This marine gastropod mollusk could be found from the Jurassic to the Cretaceous period.
A natural phenomenon occurred, causing millions of mollusks to settle down on a small area of the seabed. Those shells, together with great pressure, heat and millions of years metamorphosed into marble, a very much appreciated limestone.
The peculiarity: being so heterogeneous, each piece is unique.