What is interesting about the manual alphabet is that it is a form of Signing Exact English but contrary to popular belief, is cannot be used when one does not know the specific sign for a given word or concept, or when there does not exist a sign for it. Since fingerspelling it would be giving an English word instead of a ASL concept, only a billingual deaf person (one who is fluent in both English and American Sign Language) would understand you.
There is a way to describe concepts you do not know or concepts that do not have signs to a deaf person using ASL: it is by using classifiers: descriptive signs indicating size, consistancy, movement, etc.