Part 1: Carats and Karats
A 1.00 carat engagement ring, 18 karat gold necklace, 24 karat gold bracelet and 1.50 carat sapphire earrings; in jewellery sometimes we get caught up in the jargon of our industry. We use abbreviations, acronyms and words that we know so well that we forget that not everyone understands what we are talking about.
So what exactly is a karat? Or is it a carat? Well depending on how it is used, both are correct. A carat is generally used to describe the weight of gemstones, while a karat is used to indicate the purity of gold. With that being said, in British English ‘carat’ can also be used to indicate the purity of gold.
This is going to be a two part series. In Part 1, we are going to explore what a carat is in terms of gemstone weight. In Part 2, we will look into a karat for the purity of gold as well as the etymology of the word. The reason for two parts- I am a big believer in blogs not being overly long. Most of us read them on our phones so bite sized bits of information are best. Therefore, without further ado let us look into a carat.
A carat is a unit of measurement used for the weight of a gemstone. 1 carat, written as 1.00ct, is equal to 0.200 gram (g) or 1/5th of a gram or 200 milligrams (mg). There are 5.00ct in 1 g and there are 5000ct in 1 kg.
Now you know what a carat is, you walk into your favourite shop and hear the jeweller say “this diamond is 70 points”. Wait, what? What is a point? You think to yourself, “I thought diamonds are measured in carats?!” When a gemstone (most often a diamond) is under 1.00ct it is often described in points. There are a 100 points in 1.00 carat. A 70 point diamond would be 0.70ct. You might also hear the description of 1/2 carat, that is just as it sounds a half of a carat or 50 points.
Next you are presented with an emerald, sapphire and ruby of the same size and shape. You look at the gemstone reports and see that each have a different carat weight. You say “why are they so different?”. The jeweller explains that different gemstones have different densities.
Think about how 1 kg of muscle is smaller than 1 kg of fat. Both have the same mass, yet muscle has a greater density than fat. The emerald, sapphire and ruby each have different densities.
Relative density for gemstones is expressed via specific gravity. Taking it back to physics class, specific gravity is the “ratio of the density of a substance to that of a standard substance.” (Encyclopedia Britannica) The standard substance used in measuring the specific gravity of gemstones is water.
Without getting tangled in too many technical weeds, a gemstone’s specific gravity can be obtained with a calculation of a gemstone’s weight in air as well as its weight in water by using a hydrostatic balance. All we really need to know is that different gemstones have different specific gravity. A 1.00ct gemstone with a higher specific gravity will be smaller than a 1.00ct gemstone with a lower specific gravity.
Let’s look at the average specific gravity of the three stones mentioned above:
Emerald: 2.67-2.78
Sapphire: 3.95-4.03
Ruby: 3.97-4:00
Source: International Gem Society
Emerald has a lower specific gravity than sapphire and ruby. Therefore, the emerald would have the lowest carat weight in comparison to the ruby and sapphire. The ruby and sapphire would be almost identical as their specific gravities are almost identical. Who knew, physics could be so sparkly?
Photo 1: Hand holding a diamond with weight scale, tweezers and a pile of diamond. Worker measures the weight of gems on a jewelry scale isolated on white background by Nut Korprsisawat
Photo 2: Blue Sapphire ,green emerald,red ruby ,Square Octagon Jewelry gems Stones set on white isolate by STUDIO492