A 6mm stone would be a.75 carat diamond.
8.5 g = 42.5 carats.
Zero carats means that there is no weight to the stone. Perhaps that the stone is not a diamond? There is no notation for a diamond equal to zero carats.
The carat is a measurement of weight of precious stones. A stone of 18 carats is twice the weight of a 9 carat stone.
There could be if the stone in question weighed 100 carats.
This stone may be in the .50 carat range, but a gemologist can weigh the stone for you and give you the carat weight.
If the diamond is a round cut, a stone weighing a little over 3 carats would have the 9.5mm measurement at the girdle.
Any stone would be harmful
The mm size of a diamond stone relative to its weight in carats is directly related to the cut and the excellence of the cut. For example, a round brilliant stone measuring 8 mm in diameter at the girdle weighs about two carats. A different cut will have a different weight in carats.
Carats describe the weight of a stone. A jeweler can show you the scale used for this purpose.
1.25 carats
A jeweller can estimate the carat size of a stone, however, to learn its true carat weight, a stone must be removed from its setting and weighed.