A slang term of endearment used by a black person to describe a white child who was positively influenced by their interaction...historical relationship.
The relationship may be from a black caregiver, teacher, or simply by a white child who spent a great amount of time with a black mentor.
Origin comes from black graphite being turned into a diamond under great pressure...in this case heavy influence on the child's life.
"Bobby is a white diamond!"
"Ruby loved that white diamond!"
"Bobby loved Ruby; he was her white diamond"