Size is not everything when it comes to diamonds, far from it.
I very much doubt that her diamonds came with a Diamond Report from the International Gemological Institute......yes that is a real thing. My diamond has a numbered certificate from Antwerp which covers carat weight, colour grade, clarity grade, cut grade, polish symmetry and very precise measurements. Like most diamonds mine has a flaw, not visible to the naked eye, the report has two diagrams showing the top and bottom of the diamond and indicating where the flaw occurs.
The certificate is in effect a passport for the diamond. I know the provenance of my diamond, when it was bought there was a lot of publicity around blood diamond and how unethical the diamond trade can be.
Like many things in Rebecca's life I suspect those diamonds are style over substance.