Atoms and molecules of most solid substances are oriented in a regular order specific for the individual substance. Reason are attracting forces acting between the molecules. The final order of molecules within a solid is characterized by a minimum of resulting energy of all molecule bindings. This order is the crystallin structure of a solid.

Only significant energy transfer can cause atoms or molecules within a crystallin structure to leave from their positions. Usually this is the case when a crystallin solid starts to melt and usually this energy is transfered by heating.

Some materials can form large single crsytals under optimum conditions, when one single crystal seed is available where atoms start to deposite with the orientation of the crystal.

Some materials can be solid without forming crystals. They are designated "amorphous" materials. SiO2 which usually forms quartz crystals can condense amorphous under specific conditions and if additional substances are added which prevent crystallisation. The result is regular glass.
Some plastic materials can be crystallin if the melt is cooled so slowly that polymers have sufficient time to orient into crystallin structures or can be amorphous if the freezing process is very fast.

Amorphous plastics are transparent (polymer glas)