Finding a cleavage fracture in a gem is a sign of significant weakness. The other most common type of inclusion are other minerals. These can be small bits of debris or miniature, whole crystals. Afghani peridot sometimes has clusters of tourmaline that look like flies. Spinel can be included with tiny spinel crystals arranged in a plane.
Some mineral inclusions are opaque.
Their size and number have to do with how much they will interfere with the passage of light. Other inclusions are transparent. If they are significantly different than the surrounding crystal, they will jump right out at you. In other cases, like spinel in spinel, they are nearly invisible. This is a pattern of very slender, thread like, crystals.