If you have a certificate, you should check its authenticity. Some certificates are forged. Certificates are all fake, so the jewels are hard to be real.
If it proves to be true, it may be that there is a certain gap between the real thing and the photo. It is normal for jewelry to have small cracks and large color difference, and the price difference is one third to one half.
Of course, it does not rule out the possibility that suppliers have special channels, relatively cheap sources of goods and want to be word of mouth. It is normal that the price difference is two or three times in different levels of jewelry market. In fact, the prices of counters in shopping malls and wholesale markets are quite different.