Calcium Carbonate Crystals can appear in various conformation, in the picture are visible two very different one
<a href="//commons.wikimedia.org/w/index.php?title=User:Giorgio_Gasco&amp;action=edit&amp;redlink=1" class="new" title="User:Giorgio Gasco (page does not exist)">Giorgio Gasco</a>
CC BY 4.0
<span class="int-own-work" lang="en">Own work</span>