Gemstone Size & Weight Chart

Why Gemstone Weights Vary by Type

Unlike diamonds, which have a single specific gravity (3.52), coloured gemstones span a wide range of densities. This means two stones of identical millimetre dimensions will have different carat weights depending on the gem species. For example, a 7×5mm oval ruby (SG 4.00) weighs approximately 1.10 ct, while the same size emerald (SG 2.72) weighs only about 0.75 ct — a 32% difference.

This is why it's important to use gem-specific charts rather than a generic mm-to-carat table. Jewellers and buyers who rely on diamond weight charts for coloured stones will consistently over- or under-estimate the carat weight.

Calibrated vs Non-Calibrated Gemstones

Calibrated stones are cut to precise standard millimetre dimensions (e.g. 6×4mm, 7×5mm, 8×6mm) to fit commercially available mountings. They are more expensive per carat than non-calibrated stones because the cutter must sacrifice more rough material to hit exact dimensions. Non-calibrated (or free-size) stones are cut to maximise weight retention from the rough and require custom settings.

For production jewellery, calibrated stones save significant time and cost on mounting. For bespoke pieces, non-calibrated stones can offer better value and more unique character.

Mohs Hardness & Wearability

The Mohs hardness scale rates scratch resistance from 1 (talc) to 10 (diamond). For everyday rings, a hardness of 7 or above is generally recommended. Stones below 7 — such as opal (5.5–6.5), tanzanite (6–6.5), and peridot (6.5–7) — are better suited for earrings, pendants, and occasional-wear rings with protective settings.

Hardness is not the same as toughness. Emerald, while rated 7.5–8 on Mohs, is relatively brittle due to its natural inclusions (called jardin). Jade, rated only 6–6.5, is actually one of the toughest gemstones due to its interlocking crystal structure.

Specific Gravity in Gem Identification

Specific gravity is a key diagnostic property used by gemologists to identify stones. It can help distinguish visually similar gems — for example, blue topaz (SG 3.53) from aquamarine (SG 2.72), or red garnet (SG 3.95) from ruby (SG 4.00). The hydrostatic weighing method, where a stone is weighed in air and then in water, remains one of the simplest and most reliable identification techniques.

Need Help Sourcing Gemstones?

Casa Jewels provides specialised gemstone services including sourcing, cutting, recutting, and calibration. We work with commercial to precision-grade quality across all major gem types. Get in touch →

Also see our Diamond Shapes & Size Chart and Metal Weight Calculator.