When writing an essay about a quotation, it is helpful to follow the A.C.E. model.
First, you are going to A.nswer the question of what the quote actually means. How is knowledge a treasure? By calling it a treasure, you know that it is a thing of value, but in what way? How do you practice (use) knowledge? What relevance does a key have to a treasure or treasure chest?
Next, you will C.ite evidence or examples, either from your own life or other situations that you know of. For an essay the length that you describe, I would try to include two or three examples as evidence.
Examples serve no purpose, though, without an E.xplanation of how each of those examples illustrate the meaning of the quote. The explanation is where you use your examples to "prove" the truth of the quotation.