Pyrite Penetration Twin
Navajun, Spain
Size: 4.2x3.7x3.3cm, 2.6x2.4x2.3cm

Two crystals of the same mineral crystallize together in such a way that they are joined at a common symmetry or crystallographic plane forming a twinned crystal. When two parts of the twinned crystals grow into each other, they are called penetration twin.