The diamond weighs .125 carats. Its mm measurements depend on the cut.
An eighth of a diamond carat, if it is a round cut, measures about 3.2mm at the girdle.
The value of a diamond depends on its cut, its clarity, its colour and its carat weight. A local jeweler can give you the answer you want.
One carat weight of diamond weighs .20 grammes, so your diamond is about half that weight, or roughly half a carat.
A 1.00 point diamond is one carat, so a .12 point diamond is a little over 1/10th of a carat.
A diamond is valued by its cut, clarity, carat weight and colour. A local jeweler can show you stones in this category.
A one carat diamond in the round brilliant shape measures aproximately 6.5mm at the girdle. If the diamond is another cut, measurements will be different.
You've given the carat weight of a diamond. 'Big', then, is based on the cut of the stone.
Carat is a weight, so its weight is one tenth of a carat or one tenth of .20 grams.
If the diamond is an ideal emerald cut with these measurements, the diamond may weigh about one quarter of a carat. Otherwise, you have stated the diamond's 'big' size in measurements.
Depending on the cut, a five carat diamond always weighs five carats. If the cut is round brilliant, the mm measurement at the girdle is about 11.
A forty-point diamond is nearly half a carat, which would be fifty points.
The total weight of all the diamonds combined in the carat total weight equals one half carat -- .50. The mm measurement of each individual diamond does not depend on the total carat weight of the collection.