What is the difference between positive crystals and negative crystals?

Normally crystals do not have electrical charge.

A few types of crystals (e.g. quartz) will develop an electrical charge across them when placed under mechanical stress. This property is called piezoelectricity. But the positive and negative charges are on opposite sides of one crystal, not on two different crystals.

Perhaps you were actually thinking of something that has nothing to do with science or electricity when you asked this question.