The two measurements you give are not related, essentially.
The MM measurement could be the diameter of a brilliant cut diamond, or one measurement of another shape. The carat weight you give is a weight measurement of gemstones.
An 83 carat diamond is a rare stone, an immensely valuable stone, and probably one of a size that no human anyone knows would have a hand large enough to wear it. You can browse the list, below, to find large stones and see their other physical measurements which are determined by each stone's cut
Although 90% of women never get a diamond over 1 carat, there are many sizes and shapes that diamonds can be cut into. Jewelers can cut a diamond to any size or shape. The largest cut diamond in the world is the Centenary, which holds the record at 273 carats. Before it was cut, it weighed 520 carats.
The Hope probably originated in Golconda, India, with an undocumented provenance until it was sold by Jean Baptiste Tavernier to French Royalty in the 1600s. According to Wikipedia: "Though [Tavernier] is best known for the discovery and sale of the 118-carat (24 g) blue diamond that he subsequently sold to Louis XIV of France, was stolen in 1792 and re-emerged in London as The Hope Diamond..." You can read more about its physical characteristics and its ownership history, below.
The largest natural diamond found to date weighed more than 3,000 carats, and fit into the palm of a grown man's hand.
The world's largest cut diamond is an unnamed Fancy Black, containing small red diamond crystals. It weighs 555.55 carats and was polished into 55 facets over several years and completed in June 2004.source: Guinness Book of World RecordsAnother AnswerThe largest diamond found to date weighed more than 3,000 carats and fit into the palm of a grown man's hand. It is the Cullinan diamond.
You've given the carat weight of a diamond. 'Big', then, is based on the cut of the stone.
A one-third carat diamond weighs one-third of a carat. If the stone is a round brilliant cut stone, it's diameter measurement at the girdle will be about 4.4mm.
Your answer depends on what property interests you: clarity or carat weight, or whether or not your 'big natural...stone' is a diamond. A local jeweler can help you understand the stone that you take in to be tested.
Depending on the cut of the stone -- a round brilliant cut measures 8.2 mm at the girdle -- you can find out about how large a stone appears to be by exploring the Web site, below. (Precisely, a two carat diamond weighs two carats: that's how big it is.)
A 10 carat diamond weighs 10 carats. Apparently, you want to know how a 10 carat diamond might look on the 'big-ness' scale. If the stone is a brilliant cut, it will be around 10MM at the girdle. Explore the link below to see the visual sizes of other cuts and carat weights.
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.
In terms of a round brilliant diamond, a properly cut stone should measure approximately 5.2 mm in diameter.
A common misconception. Carat is nothing to do with size. Carat is related to weight and therefore a 1 carat stone depends on the gemstone in question and its cut. 1 carat is 0.2g and a 1 carat round-cut diamond is approximately 6.5mm in diameter. Similarly, since CZ is a diamond simulant, a 1 carat round-cut CZ is also deemed to be one that is 6.5mm in diameter but such a stone actually weighs more than 0.2g because CZ is more dense than diamond.
An 83 carat diamond is a rare stone, an immensely valuable stone, and probably one of a size that no human anyone knows would have a hand large enough to wear it. You can browse the list, below, to find large stones and see their other physical measurements which are determined by each stone's cut
If the stone is a round brilliant cut, it could weigh about one quarter of a carat. However, only a certified gemologist can give you the true weight of the stone, and certify it for you.
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.