If the diamond measures 7.5 x 7.5 mm, the diamond may weigh about two carats.
You can ask a certified gemologist to weigh the diamond for an exact carat weight.
This diamond could weigh about six carats if its measurement is 10mm x 10mm.
Carats measure the weight of the diamond, regardless of its cut. A trustworthy gemologist will remove the stone from its setting, weigh it, and remount it, for a fee.
Forty-five carats is large for any diamond. For reference, a five carat princess cut diamond measures 9.5 x 9.5 mm.
Your answer depends on the cut you are measuring. If the gem is a round cut, the carat weight will be about .10 carats.
A round cut diamond weighing .25 carats measures 4.1mm at the girdle.
If the cut is round, this weight may rate in the just under 7 carats range. If it is a different cut, the weight will be different.
Four ct tw means four carats total weight; composite may mean made up of multiple stones; princess-cut is the name of a cut for a single stone.
A round cut diamond weighing .25 carats measures 4.1mm at the girdle.
A princess cut of one carat measures 5.5mm x 5.5mm; one half carat of this cut measures 4.4mm x 4.4mm. Your diamond is about half way in between these weights, and may weigh about .75 carat. A certified gemologist can give you an exact weight.
A brief search reveals these comparisons from Blue Nile today:2.01 carats, princess cut US$10,484 - US$12,2592.01 carats, round cut, US$11,307 - US$13,309One way to read this data is to assert that one or the other cut is more expensive.However, the range of prices is based on other variables, including colour and clarity, which indicates that the cut alone does not determine the price you pay for a diamond.
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.
Your answer depends on the type of stone and the cut. The measurements you give are approximate to a sort-of princess cut, but the measurements you give are not 'true' for the ratios defined by the cut. Your local jeweler or certified gemologist can weigh the stone for you and give you its carat weight and type.