Complex sentence: Its crystals grow usually in a massive habit, though they are sometimes twinned; the twinning in cinnabar is distinctive, as it forms a penetration twin that is ridged with six ridges surrounding the point of a pyramid.
Split into simpler sentences: Its crystals grow usually in a massive habit, though they are sometimes twinned. The twinning in cinnabar is distinctive, and it forms a penetration twin that is ridged with six ridges surrounding the point of a pyramid.