When you buy a photographic print, it's not just an investment; it's a piece of art to enjoy for decades. The Certificate of Authenticity is your warrantee that you can do that. This article explains what should be on the COA and why you should care.
If you look on the back of many photographic prints, you'll see something called a "Certificate of Authenticity," or COA. Is a COA really necessary? The answer is a resounding yes.
First of all, the COA is in essence a contract between you and the artist that warrantees you are getting what you paid for. If your print is part of a limited edition, then the COA should tell you that. If it was not printed using archival paper and ink (see the article, Digital Considerations for more on this), the COA should inform you appropriately.
A good COA should have at the very least the following information:
What if there is no COA on the back of the image? Don't panic. If you are dealing with a reputable gallery, simply ask the sales representative to have the artist provide you with a COA.
One last caveat: make sure the COA itself is printed on archival paper! You don't want to have an archival image that lasts 100 years, while its COA fades into nothingness after 10 years.