Knowledge
Carat weight and visual size
The carat measures weight, not size. Why two stones of the same weight can look different in size and what to consider when comparing them.
The carat is the unit of weight for gemstones: one carat equals exactly 0.2 grams, or 200 milligrams. Weight is stated to two decimal places. It says a lot about value, but little about how large a stone looks from above.
Same weight, different size
How large a stone appears depends on its visible area from above – and that is determined by cut and proportions. A stone with a deep pavilion carries a lot of weight where it cannot be seen. A shallow stone shows more area but may lose brilliance because light escapes through the bottom.
Density plays a part
Different gemstones have different densities. A ruby (corundum), at around 4.0 g/cm³, is considerably denser than an emerald at about 2.7 g/cm³. A one-carat emerald is therefore visibly larger than a one-carat ruby. Comparisons by weight only make sense within the same species.
Reading the dimensions
Laboratory reports give dimensions in millimetres – for round stones as minimum and maximum diameter by depth, for other shapes as length by width by depth. These figures show how large a stone really is. Together with the weight they also reveal whether a stone is cut deep or shallow.
Magic thresholds
Prices often jump at round weights such as 1.00 or 2.00 carats. A 0.98-carat stone can look barely smaller than a 1.00-carat stone yet cost noticeably less. Conversely, stones are sometimes cut to just reach a threshold at the expense of their proportions.
What to look out for
- Always consider weight and dimensions together
- Compare by weight only within the same species
- Do not sacrifice cut quality and brilliance to weight
In brief
The carat measures weight; visible size depends on cut, proportions and density. Compare weight and dimensions together and only within the same species. A well-cut stone often looks more beautiful than a heavier one with poor proportions.
At NOBILITH
Every dossier states weight and dimensions from the laboratory report. This makes it possible to judge the visible size before holding the stone.
Sources
- GIA: Gem Encyclopedia, carat weight
- CIBJO: The Gemstone Book, units of measurement
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Updated 2026-10-06