Diamond type
Diamond type is a method of scientifically classifying diamonds by the level and type of their chemical impurities. Diamonds are separated into four types: Type Ia, Type Ib, Type IIa, and Type IIb. The impurities measured are at the atomic level within the crystal lattice of carbon atoms and so, unlike inclusions, require an infrared spectrometer to detect.
Diamond clarity
Diamonds become increasingly rare when considering higher clarity gradings. Only about 20% of all diamonds mined have a clarity rating high enough for the diamond to be considered appropriate for use as a gemstone; the other 80% are relegated to industrial use. Of that top 20%, a significant portion contains an inclusion or inclusions that are visible to the naked eye upon close inspection. Those that do not have a visible inclusion when the gem is examined approximately 6 inches from the naked eye are known as "eye-clean", although visible inclusions can sometimes be hidden under the setting in a piece of jewelry. The most expensive gem diamonds fall within the VS and SI grades with FL, IF, and even VVS stones commanding significant premiums. FL and IF stones are sometimes referred to as "museum quality" or "investment grade" to denote their rarity although the term "investment grade" is misleading as diamonds have historically been illiquid and questionable stores of value.