For something to be crystallized, it must be intact and resistant to change - imagine a diamond, rock salt, or rock candy. In psychology this typically is a reference to a type of human intelligence: crystallized intelligence.
Proposed by Raymond Cattell, crystallized intelligence is the overall learned knowledge and experience we acquire as we age. For example, vocabulary and facts are examples of crystallized intelligence. This differs from fluid intelligence (hyperlink?) which is a type of 'on your toes' thinking such as being able to solve problems, identify patterns, and use logic to meet new challenges. Typically, crystalized intelligence increases which age while fluid intelligence decreases with age.