Diamond Clarity explained in terms of types of diamond inclusions, black tables and milky diamonds. 

Crystal (Xt)-A mineral crystal contained in a diamond. Most included crystals are smaller diamond crystals, and usually appear white or transparent. They might also be crystals of other minerals, such as garnet or peridot, that appear colored. A crystal can create stress, causing a fracture to develop around it inside the diamond. Knot (K)-An included diamond crystal that extends to the surface after fashioning. Knots can be transparent, bright, or white. In reflected light, the outline appears dark where the knot breaks the surface. Most colored crystals are non-diamond mineral inclusions, so a dark or colored crystal that reaches a polisheddiamond's surface is classified as a crystal rather than a knot. Cavity (Cav)-An opening on the diamond's surface that occurs when part of a feather breaks away or when a crystal drops out or is forced out, typically during polishing. Evidence of a feather often remains after it breaks away. When a crystal is pulled out, the cavity resembles the original shape of the crystal. The opening of a cavity appears dark in reflected light.

Cavities are much more common than knots. Crystals on the other hand are the most common internal inclusion.