Crystals are grown for both scientific purposes and for experimentation in labs or science fairs, to demonstrate the various properties of different kinds of crystal formation. In all kinds of formation, the constant temperature plays an important role in crystal development, affecting not only how fast the crystals develop but the ways in which they grow.
Crystals typically form as a result of some kind of chemical reaction or evaporation. Water or a similar liquid will evaporate and leave behind mineral particles that will slowly bond together and grow to produce a crystal structure. This occurs in common crystals such as sugar or salt. More complex crystal structures are the result of pressure or catalysts that force atoms to assume crystalline structures.
For common crystals that are affected most strongly by evaporation, temperature is very important. The higher the temperature, the warmer the crystal solution will be, and the faster its molecules will move. This movement allows them to evaporate more quickly, leaving particles behind to form into crystals. This means that a warm solution or a warm room will allow crystals to form more quickly than a cold room.
Humidity is also very important when it comes to evaporation--solutions will evaporate more quickly in a dry environment than a humid one. Cold environments tend to be more dry than humid environments: this is why crystal formations may grow faster in the beginning in a refrigerator, but lack the strength and consistency of crystals grown in a warmer, more humid room.
Temperature typically only affects slow-growing crystals if it is steady for a long period of time. Rapid changes in temperature do not affect the crystal, unless they act upon the element itself, heating it or cooling it down until it can develop crystal formations. But changing air temperatures are unlikely to affect the rate of evaporation-based crystals. It takes time for the temperature to affect all the factors necessary to change the growth of the crystals.
In the growth of more complex crystals, the temperatures of the solution can become very important. In some cases, three elements must be raised to a certain temperature before they all combine to form the proper crystal. At a lower temperature, only two of the elements may make the crystal, instead of all three. This is why scientists carefully monitor the temperature when growing custom crystals like silicon.