Pls educate me on certs. I thought all diamonds were certified.

I''ve been doing my due diligence searching for that diamond that will knock my socks off. I''ve been to Tiffany & Co., 47th Street - NYC, a couple of local jewelers, Whiteflash, GOG, etc. I visited Michael''s Jewelers today to take a look at the Hearts on Fire stones. At the same time, I was looking at some non-branded stones and came across one that I thought was beautiful. Round, 1.75 ct, VS2/D, but that''s all I know & I wanted more info. I asked the salesperson if I could see the cert and she said they don''t keep them on hand. I asked her if she could get it for me & she was hesitant. I asked her if it was AGS or GIA certified, and she said many of their diamonds are certified by Michael''s. This is where I need some education. I know that Tiffany has their own in-house lab and doesn''t use GIA or AGS, but I thought it was because they were Tiffany. I didn''t know that a mall-store like Michael''s also had their own in-house lab. I thought that most diamonds, at least those being sold in stores, would all come with some kind of cert from an independent lab. After pushing her just a little bit, she said she was able to confirm that this stone did come with a GIA cert, but she''d have to have them fax it to her & I was fine with that. She just called to say it would be faxed tomorrow & I can come into the store to take a look at it & get whatever info I need off of it. But, how does diamond rating work?