Diamonds found in nature have the following characteristics that can be used for field testing: Occurs as octahedral (cubic), rounded crystals.  Can be colorless, yellow, brown, gray, green and black due to the presence of impurities.  Optically, pure diamonds are transparent with adamantine luster (high index of refraction) and strong dispersion of light.  Very hard, ranking 10 on the Moh's scale. Has a density close to 3.5 grams per cubic centimeter. Exhibits perfect cleavage, showing crystalline planes.  Insoluble in hydrochloric acid or nitric acid. Infusible, because diamonds have a high melting point. Diamonds burn with difficulty, giving off carbon dioxide only at temperatures above 800 degrees Celsius, under ambient pressure.