Definition of niacin for English Language Learners. : a type of natural substance (called a vitamin) that is found in certain foods and that helps your body to be healthy.
Red diamonds are the most expensive in terms of money per carat, since red is the rarest colour of natural diamonds.
Elegant and noteworthy this rare blue diamond ring is a Bulgari custom-designed ring from a European collector during the 1960s. The owner gave the ring to his wife at an estimated value of $2.9 million. It is considered to be one of the most expensive jewels in the world.