This is a term that is always the same. So it is a fact that 1ct weight 0,20gr. It does not mean that everything that is bigger is also more valuable, but in this case it is. A diamond in the piece, which, for example, weighs 2ct has a higher value than 4 diamonds weighing 0,5ct simply because in nature it is rare to find big diamonds. Diamonds are measured to an accuracy of two decimal places, you should not be surprised if the certificate might say Ct 0.15 or 0.33 or 1.23. Every jeweler who sells diamonds or other precious stones is obliged to issue a certificate of its fineness, purity, price, stamped and signed obligated with full responsibility.