A coating is what makes a mirror a mirror. It increases the reflectivity from under ten percent to around ninety percent of the light striking it.
There are three types of coating that are feasible for amateur mirrors:
Aluminum which often costs over forty dollars to coat a six inch mirror,
Beral or beryllium is the standard coating from C.L. Clausings and costs thirty dollars for a six inch mirror, it should last a long while and is as reflective as aluminum ,
Silver, is the most reflective but it lasts the shortest amount of time and needs to be re-done sometimes as frequently as every few weeks. The advantage of silver is that it is the only coating that can be applied at home, though it is a bit messy, it would probably not be hard to do if you have a lab available.
Gold and other exotic coating are also available but are usually only needed in very specific situations such as infrared observing. (CJ has a gold coated mirror).
The picture is of a coated mirror reflecting a lighting fixture
