Metals are a class of inorganic crystalline materials composed of either a single element or a blend called an alloy. Common metals and metal alloys include: aluminum, copper, nickel, steel, brass, silver, and titanium. Bare metals are most readily processed using a 1.06 micron fiber laser with some metals also absorbing, to a lesser extent, a 10.6 or 9.3 micron CO₂ laser wavelength. Laser marking of metals is most easily achieved using a 1.06 micron fiber laser to heat the surface, causing oxidation in ambient air to form a permanent mark. Additional processes can also be used on some metals to roughen the surface, creating a light mark. Laser engraving of metals is performed with a fiber laser with repeated passes used to achieve increasingly greater depth. Laser engraving in metal with a fiber laser is typically rough and oxidized and can range from lighter to darker in appearance depending upon the material processed. Laser cutting of metals with a fiber laser is possible. Laser marking of ferrous metals (those containing iron) and titanium is also feasible with either a 10.6 or 9.3 micron CO₂ laser and an HPDFO™ (High Power Density Focusing Optic) lens. The high power density of the lens allows the less efficiently absorbed CO₂ laser energy to heat the surface of the metal, causing oxidation in ambient air to form a permanent mark.