Diamond
Sherlock was like a diamond. His brilliance sparkled, enthralling everyone who encountered it. It made most people envious, made them lash out in anger, assuming that the hardness of his exterior couldn't be hurt by anything.
But then there was John.
John treasured Sherlock, admiring his intellect, telling him he was amazing. He saw past Sherlock's cold facets and saw the times Sherlock was hurt, the times he lost his confidence. And John took care of him, soothed the aches, restored his lustre.
John treasured the detective above all else, for he knew that Sherlock was one of a kind.