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.
What makes the blue garnet that much more beautiful is its color changing property, which is due to high amounts of the metal vanadium. In daylight, the gem is a soft blue-green but changes to a vivid purple under incandescent light. The most expensive blue garnet, at 4.2 carats, sold for $6.8 million in 2003.