I have a sapphire pendant necklace that came with a certificate from something called the AGTA. Is this a reliable gemological institution?

I have only ever heard of the GIA when it comes to grading diamonds and gemstones, and when I was shopping for wedding rings for my husband and myself, I remember lots of jewelers telling me that the GIA was the only reliable authority for diamond certification. Does the same go for colored gemstones that aren't diamonds?