Boring question #67: How do you know the appraiser has given you the truth about your gem?

For the sake of argument, we will use a diamond. You have a loose stone and you want to know it’s value. Since you do not know about diamonds, you go to have it appraised. The appraiser knows you know nothing about diamonds because that is why you are there in the first place. What stops the appraiser from lying to you, especially if you are looking to sell the gem? As said, you know nothing about the value and they know it so they can devalue the worth and you would never know. I guess you could take it to another appraiser and compare the price but that is more time and more money just to find out what a gem is worth. How do you know the price you are quoted is a legitimate, honest price?