The strict answer is that that a 15 carat stone weighs 15 carats. You probably want to know how big the stone looks.
Use the link, below, to get a visual idea of about how large a stone might look depending on its carat weight, and its cut.
If the 15 carat stone is a brilliant cut, the stone would be about 16.3 MM across 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.
The diamond weighs 112 carats, or 0.790136 ounces.
A 2 mm round diamond weighs about .03 carats, so a 1.5 mm round diamond would be smaller than .03 carats. A .01 carat diamond is about as big as a grain of sand.
A 'big diamond' is one valued high enough that you are uncomfortable wearing it in public or leaving it in a jewelry box on your dresser. The value of this diamond means that you only wear it when you feel safe, and when you're not wearing it, you keep it in a safe. 'Big' is always relative.
An eighth of a diamond carat, if it is a round cut, measures about 3.2mm at the girdle.
15ct or .625
Between 1854-1931 is the only time 15ct gold was an official standard
The Diamond as Big as the Ritz was created in 1922.
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.
You've given the carat weight of a diamond. 'Big', then, is based on the cut of the stone.
625 stamp means the item is 62.5 % gold, which = 15ct
No real size difference is important. A tiny diamond is still a diamond.
If you mean diamond & Pearl, then in Diamond, Dialga and in Pearl, Palkia
The diamond weighs 112 carats, or 0.790136 ounces.
A 2 mm round diamond weighs about .03 carats, so a 1.5 mm round diamond would be smaller than .03 carats. A .01 carat diamond is about as big as a grain of sand.
No matter how big the weight it will never be able to crush the diamond.
A 'big diamond' is one valued high enough that you are uncomfortable wearing it in public or leaving it in a jewelry box on your dresser. The value of this diamond means that you only wear it when you feel safe, and when you're not wearing it, you keep it in a safe. 'Big' is always relative.