Different types of drill bits are used to cut holes in different types of materials. Diamond drill bits are used for materials such as glass and tile; black oxide bits are common for plastics and carbon. Cobalt drill bits are particularly successful in cutting through hard metals such as stainless steel and cast iron, although they can be used for softer materials, too. Cobalt drill bits are not made of pure cobalt, but rather a steel alloy with 5 to 8 percent cobalt. The 5-percent alloy is known as M35 grade, and the 8-percent alloy is M42. The cobalt increases the strength of the steel and makes it more heat-resistant; this is an important factor in drilling hard materials because the friction of metal against metal can produce high temperatures that damage the material or the drill bit. Bosch, for instance, makes a cobalt drill bit that can withstand temperatures of up to 1,100 degrees Fahrenheit (593 degrees Celsius).
New cobalt drill bits are a dull gold color, making them distinctive on the shelf (or in your toolbox). The color occurs when the drill bits are baked in the process of production; it’s not a paint or plating -- cobalt drill bits are cobalt alloy through and through. For this reason, they can be sharpened relatively easily with cutting fluid while retaining their strength and durability (although the gold color may wear off).
Cobalt drill bits are often more expensive than other drill bits, but if you regularly drill through metal, they are a good investment since they tend to last longer. You can buy sets of drill bits in graduated sizes or individual drill bits for your specific needs.