the luminescence colors is often unclear and not unique. Blue emission from type IIa and IIb diamonds is reliably identified with dislocations by directly correlating the emission with dislocations in an electron microscope. However, blue emission in type Ia diamond could be either due to dislocations or the N3 defects (three nitrogen atoms bordering a vacancy). Green emission in natural diamond is usually due to the H3 center (two substitutional nitrogen atoms separated by a vacancy), whereas in synthetic diamond it usually originates from nickel used as a catalyst (see figure). Orange or red emission could be due to various reasons, one being the nitrogen-vacancy center which is present in sufficient quantities in all types of diamond, even type IIb. Optical absorption Cape series (Ia) diamonds have a visible absorption spectrum (as seen through a direct-vision spectroscope) consisting of a fine line in the violet at ; however, this line is often invisible until the diamond has been cooled to very low temperatures. Associated with this are weaker lines at , , , , and . All those lines are labeled as N3 and N2 optical centers and associated with a defect consisting of three nitrogen atoms bordering a vacancy. Other stones show additional bands: brown, green, or yellow diamonds show a band in the green at (H3 center, see above), sometimes accompanied by two additional weak bands at and (H4 center, a large complex presumably involving 4 substitutional nitrogen atoms and 2 lattice vacancies). Type IIb diamonds may absorb in the far red due to the substitutional boron, but otherwise show no observable visible absorption spectrum. Gemological laboratories make use of spectrophotometer machines that can distinguish natural, artificial, and color-enhanced diamonds. The spectrophotometers analyze the infrared, visible, and ultraviolet absorption and luminescence spectra of diamonds cooled with liquid nitrogen to detect tell-tale absorption lines that are not normally discernible. Electrical properties Diamond is a good electrical insulator, having a resistivity of to ( – ). Most natural blue diamonds are an exception and are semiconductors due to substitutional boron impurities replacing carbon atoms. Natural blue or blue-gray diamonds, common for the Argyle diamond mine in Australia, are rich in hydrogen; these diamonds are not semiconductors and it is unclear whether hydrogen is actually responsible for their blue-gray color. Natural blue diamonds containing boron and synthetic diamonds doped with boron are p-type semiconductors. N-type diamond films are reproducibly synthesized by