If you buy a diamond over one carat we strongly recommend it be accompanied by an independent grading report (often called a certificate or cert), like these shown here.
Grading reports don't tell you how much a diamond is worth, they give an independent expert opinion on its quality and can only be issued for a loose stone. These grading institutes do not sell diamonds and the better, stricter labs are recognized worldwide. Diamonds with these reports trade for a few percent more.
But diamond grading is subjective, and therefore never 100% consistent among even the top graders. So different reports on the same diamond, even from the same lab may vary.
EGL and some Israeli Labs are seen in the industry to be " soft labs". Australia has two labs with good reputations based in Sydney, DCLA and GSL. Valuations go out of date, but unless the diamond is damaged, the cert is good forever. Store it in a safe place.
Do not confuse a cert with a valuation. A valuation is an opinion of the value of the diamond or piece of jewellery and should be done by a JAA or JATVC registered valuer.
Precious Metals provide a respected independent grading certificate with every diamond purchase above .90ct.
AGS Diamond Quality Document™
GIA Grading Report
DCLA Grading Report