Authenticity is the quality of being genuine or real. You might question the authenticity of your eccentric uncle's photo of a UFO.
The word authenticity is the state of something being authentic, or legitimate and true. Authenticity is important when the value of something is dependent on where it came from or how it was made. If you buy a ring said to be worn by Marie Antoinette, you will likely pay a hefty price for it. Before you spend all of that money, you should consult with a historian or experienced jeweler, who will be able to verify its authenticity.