How Are Jewelry Measurements Explained? A Simple Guide
Jewelry is a significant and meaningful investment for many people, often tied to important milestones like engagements, weddings, and family heirlooms. When buying, selling, or insuring jewelry, it's essential to understand the terms used to describe gemstones and precious metals, as they affect both value and desirability. Several units of measurement—such as carat, karat, millimeter, and others—are commonly used in the jewelry industry, each with specific meanings and uses. Let’s review the most common terminology and measurements to make your next trip to the jewelry store a little less stressful.
1. Carat (ct)
The term carat refers to the weight used to measure gemstones, especially diamonds. A carat equals 200 milligrams and is the most common way to refer to the size of a stone. But it’s also important to understand that carat weight doesn’t necessarily equate to the same dimensions of every stone. Depending upon the type of stone and cut, dimensions can vary. A 1.00 carat diamond will have different dimensions than a 1.00 carat ruby, for example. 
The term "carat" comes from the carob seed. It was originally used by the Romans as the standard to weigh gemstones because it was thought that carob seeds were uniform in weight, and therefore considered a reliable way to reference the weight of stones in a consistent manner. In 1907 the carat was officially standardized as exactly 200 milligrams. This made for a consistent system of measurement across different countries.
2. Karat (kt)
Karat, which is often confused with carat, is not used to measure the weight of gemstones, but instead defines the purity of gold. Gold in its purest form, 24 karats (24kt), is a very soft and malleable metal, which makes it unsuitable for everyday wear. To create durable, wearable gold jewelry, pure gold is alloyed with other metals in specific percentages, forming lower-karat options like 10kt, 14kt, 18kt, and 22kt gold. The higher the karat, the softer and more expensive the gold. The word "karat" also comes from the carob seed and was used by the Romans to define the purity of gold, eventually being adopted over time as the standard measurement.
Yellow gold, which naturally has a rich yellow hue, is made by alloying pure gold with metals like copper and silver. The exact proportions of these metals vary depending on the karat of the gold. Higher karats, containing more pure gold, display a more vibrant yellow color. White gold, by contrast, combines pure gold with white metals such as nickel, palladium, or silver and is often plated with rhodium for a brilliant white finish. The resulting color and karat depend on the specific proportions of pure gold to alloy metals. Rose gold is made by alloying pure gold with copper, which provides its signature pink hue. Other metals may be added in small amounts to adjust the shade, with each karat level bringing a different balance of pure gold to copper. Green gold, though less common, is achieved by combining pure gold with silver, copper, and sometimes cadmium. The resulting karat and shade of green is determined by the mix of these metals.
3. Gauge
Gauge can be applied to many different industries. For jewelry, gauge refers to the thickness of a piece of jewelry–or more specifically, how thin or thick metal wire (or sheets) are. This may seem counterintuitive but a lower gauge indicates a thicker metal, while a higher gauge means thinner jewelry.
4. Diamond Proportions and Percentages
When diamond shopping, you’ll notice diamond proportions and percentages on a diamond grading report. These numbers refer to the ratios that determine the symmetry and cut of the stone, which contribute to how much the stone sparkles. Ultimately these affect the diamond’s appearance and value.
Table Percentage - the table percentage is calculated by the width of the diamond’s table (the top facet) compared to the overall width of the diamond. The ideal range for this is from 53% to 57%.
Depth Percentage - the depth percentage is calculated by the height of the diamond from the table to the culet (the bottom point) compared to the diamond's overall diameter. The ideal range for this is from 59% to 62%. If a stone is too shallow or too deep, it will limit the way light can pass through the stone, affecting the light reflection and sparkle.
5. Pennyweight (dwt)
The term pennyweight (dwt) is a unit of measurement used to weigh precious metals, such as gold and platinum. It's used mostly in the United States and the United Kingdom. One pennyweight is equal to approximately 1.555 grams. The pennyweight originated from the troy weight system of the Middle Ages and morphed into the term pennyweight because it was found that 240 pennies was equal in weight to one pound of silver. It was later standardized and is still used today to weigh fine jewelry.
7. Point (pt)
The term point is used when measuring the size of small diamonds. One point equals 1/100th of a carat. For example, a 0.25 carat diamond can also be called a 25 point diamond. This system was introduced in the 19th century to address the need for more precise terminology as diamond production expanded. With the rise of mass production of smaller diamonds for use as accent stones in fine jewelry, clear and consistent language became essential.
8. Carat Total Weight (ctw)
Carat total weight (ctw) refers to the combined weight of all the diamonds or gemstones in a piece of jewelry. For example, if you have a 1.00 carat center stone and 30 side stones that each weigh 1 point, then your carat total weight would be 1.30 carats.
Now that you have a clear understanding of jewelry terminology and measurements, you can more confidently shop for your next jewelry purchase without the fear of not understanding the salesperson or feeling unsure about what you’re purchasing. From carats and karats to proportions, points, and pennyweights, each term allows you to be a more informed consumer, whether you’re picking out an engagement ring, a push present, or a gift for a friend or family member.