The term 'diamond complement' is a type of jewelry that consists of two pieces, one of which is a diamond and the other is a complementary piece. The crossword solution for this term is 'NINE, ' which refers to the number of facets on a diamond. A facet is a flat surface on a diamond that reflects light and gives it its sparkle. The number of facets on a diamond can range from four to twenty-four, but NINE is the most common number.