Gemstone specific gravity and refractive index chart
Specific gravity and refractive index are two of the first tests used to identify a gemstone. This table lists typical values for the stones dealers handle.
| Gemstone | Specific gravity | Refractive index | Mohs hardness |
|---|---|---|---|
| Diamond | 3.52 | 2.417 | 10 |
| Ruby | 3.99 – 4.05 | 1.762 – 1.770 | 9 |
| Sapphire | 3.99 – 4.05 | 1.762 – 1.770 | 9 |
| Chrysoberyl (cat's eye, alexandrite) | 3.70 – 3.78 | 1.746 – 1.755 | 8.5 |
| Spinel | 3.58 – 3.61 | 1.712 – 1.762 | 8 |
| Topaz | 3.50 – 3.57 | 1.609 – 1.643 | 8 |
| Emerald | 2.67 – 2.78 | 1.565 – 1.602 | 7.5 – 8 |
| Aquamarine | 2.68 – 2.74 | 1.567 – 1.590 | 7.5 – 8 |
| Zircon | 3.93 – 4.73 | 1.810 – 1.984 | 6.5 – 7.5 |
| Tourmaline | 3.01 – 3.11 | 1.624 – 1.644 | 7 – 7.5 |
| Garnet | 3.58 – 4.32 | 1.714 – 1.888 | 6.5 – 7.5 |
| Amethyst and citrine (quartz) | 2.65 | 1.544 – 1.553 | 7 |
| Peridot | 3.27 – 3.37 | 1.654 – 1.690 | 6.5 – 7 |
| Tanzanite | 3.35 | 1.691 – 1.700 | 6.5 – 7 |
| Jadeite | 3.30 – 3.38 | 1.654 – 1.667 | 6.5 – 7 |
| Moonstone | 2.56 – 2.59 | 1.518 – 1.526 | 6 – 6.5 |
| Opal | 1.98 – 2.25 | 1.37 – 1.47 | 5.5 – 6.5 |
| Turquoise | 2.60 – 2.90 | 1.610 – 1.650 | 5 – 6 |
| Pearl | 2.60 – 2.85 | 1.52 – 1.66 | 2.5 – 4.5 |
Specific gravity
Specific gravity (SG) is a stone's density compared with water. It lets you check an identification by weighing a stone in air and in water, and it is used to estimate weight from size.
Refractive index
Refractive index (RI) measures how much light bends in the stone, and is read with a refractometer. Many stones have a range because of double refraction and variety.
Use as a guide
These are typical ranges. Ranges overlap, and treated, synthetic and unusual stones can differ, so use a laboratory for any stone of value.
Frequently asked questions
What is the specific gravity of sapphire?
About 3.99 to 4.05, the same as ruby.
What is the refractive index of diamond?
2.417.
Can I identify a stone from SG and RI alone?
They narrow the possibilities but do not prove identity. Overlapping ranges, synthetics and treatments mean a laboratory should confirm valuable stones.