Many higher priced flawless, VS1 and rare colored diamonds are certified. In Europe we have the EGL (European Gemological Laboratory) which has often been questioned on the validity of its reports. In the USA there is the GIA (Gemological Institute of America). However, the GIA do not actually certify, they only grade the diamond and issue a report. In fact it was the GIA back in the 1940's that established the "4Cs" and the International Diamond Grading System which to this day is the worldwide standard for evaluating diamond quality. Some other countries have their own certification which normally follow the same format as the GIA. However depending on diamond size, there is a cost from $50 to $1000 per stone in obtaining these reports. Due to this and the existence of many fake certificates, top end jewellers rely upon their prestige and name to guarantee the stated grade quality of the diamond to their customers.

So how do you spot a fake diamond?
Commonly passed off as diamonds are Cubic Zirconia, White Sapphire and Rutile. These are not hard like real diamonds and scratch or chip easily. There is also Moissanite, which in fact is a very good and sometimes considered a better alternative to diamond.
PRICE. Average trade prices per carat of diamonds vary from 450 to 1400 depending the grade. So if you see a diamond ring, pendant or earring with say a 1 carat diamond for less than 500, its not a real diamond unless of extremely poor quality.
CLARITY & COLOR. Unless the diamond is 'FL' flawless, which are rare and the highest quality and price, diamonds by nature contain flaws and are graded from flawless(FL) VVS, VS, S1, l1. Anything at S1 or below you will clearly see flaws in the diamond. By nature, most diamonds have a slight yellow tint which vary from hardly noticeable to a grayish yellow. The more yellowish the diamond, the cheaper it becomes, down to virtually worthless.
SPARKLE. Diamonds are supposed to sparkle or as its termed scintillation. Apart from the grade, how the diamond is cut will determine its sparkle. Due to its structure, a diamond should sparkle clear white in natural light with very few other rainbow colors. So if your diamond is sparkling all colors of the rainbow, its not a natural diamond.
One of the oldest tests for diamonds was the water or oil test. Due to a diamonds own relationship to oil, water will immediately run off a diamond, while oil will actually stick and even soak slightly into a diamond and discolor it if left long enough.