Diamond and Jewelry Gallery Fraudulent Misrepresentation of Diamond Quality Studio City, California

I came into the store on several occasions looking for a loose diamond stone for an engagement ring. When I finally settled on a stone, I put my trust in the owner John. I asked him about a certificate for the stone, and he told me not to worry, that they are GIA diamond graders. The stone was an I near colorless and SI1 clarity  of 1.54 carats for $5800. I received a certificate of appraisal in the mail that was signed by a "GIA diamond grader".  2 weeks ago, I went to have the ring insured, and when the diamond was looked at, I was told that it was of so substantially less quality that it was worth less than $2500 if I was to sell the diamond.  I was told that the color was of M quality and clarity was lower as well. I took this information to John at Diamond Gallery, and when he looked at the stone, he told me that it was of the right quality, and that a jeweler would say anything, so he wanted to see it in writing. He stood behind the diamond 100%. I then paid over $190 to have EGL certify the diamond(I was originally going to use GIA because of the high regard for the appraisals but the turnaround time was weeks) The certification came back L color which is yellow and SI3 clarity which is only one grade below visible flaws to the naked eye. I then took the diamond and certification to John. His first reaction was an accusation that I or another jeweler had switched diamonds, but when I pressed him on his accusation he backed off. He refuses to issue a refund and seems defiant about his busines practices.