Diamond Size Chart: Carat, Points & MM Conversion Guide

Understanding Diamond Sizes: Carat Weight vs. Visual Dimensions

When shopping for an engagement ring in Hong Kong, "carat" is often the first word you hear. However, many first-time buyers confuse carat weight with the actual physical size of the diamond. Carat is a measure of weight, not dimensions. To help you make an informed decision, this guide explores the relationship between carat weight, points, and millimeter (mm) diameter, ensuring you find a diamond that looks as magnificent as you envision.

What are Diamond "Points"?

In the jewellery industry, especially in Hong Kong, you will often hear jewellers refer to diamonds in "points" (or "分" in Cantonese). This is simply a smaller unit of measurement used for diamonds weighing less than one carat. One carat is divided into 100 points. Therefore:

• 10 points is equal to 0.10 carats
• 30 points is equal to 0.30 carats
• 50 points is equal to 0.50 carats
• 100 points is equal to 1.00 carat

Understanding this terminology makes it much easier to communicate with GIA-certified diamond specialists when discussing your budget and preferences.

The Diamond Size Chart: Carat to Millimeter Conversion

Because diamonds are three-dimensional, their visual size when viewed from the top (face-up appearance) is measured in millimeters. For a classic round brilliant cut diamond—the most popular cut for engagement rings—here is a standard conversion guide showing how carat weight typically translates to diameter:

• 0.30 Carat (30 points): Approximately 4.3 mm
• 0.50 Carat (50 points): Approximately 5.1 mm
• 0.70 Carat (70 points): Approximately 5.7 mm
• 0.90 Carat (90 points): Approximately 6.2 mm
• 1.00 Carat (100 points): Approximately 6.4 mm
• 1.50 Carat: Approximately 7.3 mm
• 2.00 Carats: Approximately 8.1 mm

It is important to note that this chart applies specifically to round brilliant cut diamonds. Fancy shapes like oval, marquise, or pear cuts have different proportions and often look larger face-up than a round diamond of the same carat weight.

Why Two Diamonds of the Same Carat Can Look Different

It is a common misconception that all 1-carat diamonds look the same size. The secret lies in the cut of the diamond. The cut grade, determined by laboratories like the GIA, dictates how well a diamond is proportioned:

• Too Deep: If a diamond is cut too deep, much of its weight is hidden in the pavilion (the bottom part). This results in a smaller diameter, making the diamond look smaller than its actual carat weight suggests.
• Too Shallow: If a diamond is cut too shallow, it may have a larger diameter, but light will leak out of the bottom, causing the diamond to lose its sparkle and look dull.
• Excellent Cut: A GIA Excellent cut diamond strikes the perfect balance, maximizing both light reflection (brilliance and fire) and proper physical proportions.

How to Choose the Right Size for Your Engagement Ring

Selecting the perfect diamond size involves balancing your budget, the recipient's finger size, and the ring setting. Here are a few expert tips for Hong Kong couples:

• Consider Finger Size: On a slender finger (common in Hong Kong, typically sizes US 4 to 6), a 0.50-carat to 0.70-carat diamond will look remarkably substantial. A 1-carat diamond will appear exceptionally large.
• Leverage the Setting: If you want to maximize visual impact within a specific budget, consider a halo setting. A halo of smaller diamonds surrounding the center stone can make a 0.50-carat diamond look like a 1.00-carat ring.
• Prioritize Cut Over Carat: A slightly smaller diamond with an Excellent cut will sparkle intensely, drawing more attention and looking more impressive than a larger diamond with a poor cut.

Consult a GIA-Certified Specialist

While size charts and conversion tables offer an excellent starting point, nothing compares to seeing diamonds in person under proper lighting. Comparing different carat weights and cut grades side-by-side allows you to understand how proportions affect brilliance and visual size. Working with a trusted, GIA-certified specialist ensures you receive honest guidance, helping you find a diamond that offers the perfect balance of beauty, size, and value.