what is crystal twilling ?
It is crystal twinning not twilling.
Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner.
The result is an intergrowth of two separate crystals in a variety of specific configurations. A twin boundary or composition surface separates the two crystals. Crystallographers classify twinned crystals by a number of twin laws. These twin laws are specific to the crystal system. The type of twinning can be a diagnostic tool in mineral identification.