In the vicinity of Cluj-Napoca, in the valley of the Bács stream, you can often find the remains of a sea urchin Echinolampas giganteus from the Eocene (ca. 45 million years old). In some of these, there are pale blue or colourless celestine crystals (SrSO4). Unfortunately, we can only find out the content of the sea urchins by smashing them with a hammer. Because of this, many sea urchins have fallen victim to mineral collectors, to the great sorrow of palaeontologists and fossil collectors.
In the picture: Sea urchin with celestine in it, Baciu, Romania. Size: 8 cm. Photo by Géza Kulcsár