One tenth of a carat is a small weight for a diamond -- your local jeweler may have such a diamond to sell.
Two tenth carat isn't a term usually associated with a diamond. Every diamond is priced according to its carat weight, its colour, its cut and its clarity.
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.
A 1.00 point diamond is one carat, so a .12 point diamond is a little over 1/10th of a carat.
The size of a diamond is measured in carats, with 1 carat being equivalent to 0.2 grams. A diamond with a mass of 0.10g would be a 0.50 carat diamond.
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.
The phrase "carat TW" means the carat total weight. This implies there may be multiple diamonds involved in the piece of jewelry and when they are all added together, one-tenth of a carat is their combined weight. "Carat" is just a diamond industry special word for weight of a diamond stone. One carat is equal to 200 milligrams, or in other words, a 5-carat stone weighs 1 gram. The word "carat" originates from the Greek word kerátion, which means carob beans, which were known in the ancient world for uniformity of their size and weight and were used as a measure of weight for different objects, including gemstones. In the Far East instead of carob beans jewelers used rice grains to determine the weight of gemstones. Because of this fact, you may occasionally hear some jewelers referring to a 1-carat diamond a "four grainer," which means in the past 1 carat was equal to four grains of rice. The weight of smaller diamonds is often expressed as points, not carats. One carat is equal to 100 points. Thus, for example, a 10-point diamond has the weight of 0.1 carats.
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.
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.