Understanding Opal Sizes: A Practical Guide
When shopping for opal jewellery, size matters—but not always in the way you might expect. Unlike diamonds, which are measured by carat weight, opals are typically described by their dimensions in millimetres. Understanding how opal size translates to visual impact and wearability will help you choose the perfect piece for your collection.
How Opal Size is Measured
Opal size is measured by the length and width of the stone in millimetres (mm). For oval or round stones, we note the diameter. For freeform or irregular cuts, both the longest and widest points are recorded. This measurement reflects the actual footprint of the stone on your finger, pendant, or earring.
It's important to note that two opals of the same millimetre size can appear different in visual weight depending on their thickness, colour saturation, and play-of-colour intensity. A vibrant, multi-coloured opal will often appear larger and more striking than a pale stone of identical dimensions.
Opal Shapes Explained
Opal shape is as important as size when it comes to visual impact and jewellery design. Each shape brings its own character and showcases the stone's colour play differently.
Oval is the most popular opal cut. It maximizes the stone's surface area, allowing the full spectrum of colours to display. Oval opals work beautifully in rings, pendants, and earrings, and they're flattering on most hand shapes.
Round opals offer classic elegance and symmetry. They're ideal for solitaire rings and stud earrings, creating a balanced, timeless look. Round cuts tend to concentrate colour play in the centre, making them excellent for showcasing vibrant hues.
Freeform or irregular shapes celebrate the natural character of the opal. These unique cuts are prized by collectors and designers who want one-of-a-kind pieces. Freeform opals often display the most dramatic colour play and tell a story through their organic contours.
Pear shapes elongate the hand and work beautifully in drop earrings and pendant settings. The pointed end adds an element of refinement and movement to jewellery designs.
Understanding Opal Weight and Carat
Unlike diamonds, opals are not typically sold by carat weight alone. However, carat weight does matter for valuation and rarity. One carat equals 200 milligrams, and opal density varies depending on the type and water content of the stone.
A general guideline: an 8mm round opal weighs approximately 1 to 1.5 carats, while a 12mm oval opal typically weighs 2 to 3 carats. Thicker stones and those with higher water content may weigh slightly more. Black opals, which are denser, often weigh more than lighter varieties of the same size.
When evaluating an opal's value, consider the combination of size (in millimetres), weight (in carats), shape, colour saturation, and play-of-colour pattern. A smaller opal with exceptional colour play and vibrant hues may be more valuable than a larger stone with muted tones.
| Rounds | Ovals | Pear | Marquise | ||||
| Size | Weight | Size | Weight | Size | Weight | Size | Weight |
| 1.6 mm | .02 carat | 5 x 3 mm | .25 carat | 6 x 5 mm | .7 carat | 4 x 2 mm | .10 carat |
| 2 mm | .03 carat | 6 x 4 mm | .5 carat | 7 x 5 mm | .75 carat | 5 x 2.5 mm | .25 carat |
| 2.5 mm | .06 carat | 7 x 5 mm | 1.0 carat | 8 x 5 mm | 1.0 carat | 6 x 3 mm | .30 carat |
| 3 mm | .10 carat | 8 x 6 mm | 1.5 carat | 9 x 6 mm | 1.5 carat | 8 x 4 mm | .75 carat |
| 3.5 mm | .16 carat | 9 x 7 mm | 2.5 carat | 10 x 7 mm | 2.0 carat | 10 x 5 mm | 1.0 carat |
| 4 mm | .25 carat | 10 x 8 mm | 3.0 carat | 11 x 7 mm | 2.25 carat | 12 x 6 mm | 2.0 carat |
| 4.5 mm | .33 carat | 11 x 9 mm | 4.0 carat | 12 x 8 mm | 3.0 carat | 14 x 7 mm | 3.0 carat |
| 5 mm | .50 carat | 12 x 10 mm | 5.0 carat | 13 x 9 mm | 3.5 carat | 16 x 8 mm | 4.0 carat |
| 5.5 mm | .65 carat | 14 x 10 mm | 6.0 carat | 15 x 10 mm | 5.5 carat | 18 x 9 mm | 6.0 carat |
| 6 mm | .75 carat | 14 x 12 mm | 8.0 carat | 16 x 12 mm | 8.6 carat | 20 x 10 mm | 8.5 carat |
Finding Your Perfect Match
The ideal opal combines your preferred shape and size with the colour palette that speaks to you. Whether you're drawn to the classic elegance of an oval, the boldness of a freeform, or the symmetry of a round, each shape and size offers distinct advantages. Explore our collection to discover the opal that captures your imagination.