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Opal
Opal is the least dense of the 17 gemstones here. At 2.115 it is about 80% of Quartz, and about 45% of Zircon, the densest here.
| Specific gravity | 1.98–2.25 |
|---|---|
| Mohs hardness | 5.5–6.5 |
| Composition | Hydrated silica |
| Varieties | White opal, Black opal, Water opal, Fire opal |
Data: U.S. Geological Survey, Gemstones (General Interest Publication), pubs.usgs.gov/gip/gemstones. Licensed under Public domain (USGS). Changes: Specific-gravity ranges drawn to scale and ordered by their midpoint; jade shown as its two minerals; up to three varieties named per gem. Checked 2026-09-19.
Figures for this entry: source record.
How it sits against the others
| Gemstone | To scale | Specific gravity | Mohs hardness | Composition | Varieties |
|---|---|---|---|---|---|
| Zircon | 4.6–4.7 | 7.5 | Zirconium silicate | Jargon, Matura diamond, Hyacinth | |
| Corundum | 3.96–4.05 | 9 | Aluminum oxide | Ruby, Sapphire | |
| Garnet | 3.5–4.3 | 6.5–7.5 | A group of silicate minerals | Almandine, Almandine-spessartine, Andradite, Demantoid, Topazolite, Grossular, Tsavorite, Hessonite, Pyrope, Chrome pyrope, Pyrope-Almadine, Pyrope-Spessartine, Malaia, Color-change garnet, Rhodolite, Spessartine, Uvarovite | |
| Spinel | 3.58–4.06 | 8 | Magnesium aluminum oxide | Balas ruby, Almandine spinel, Rubicelle, Sapphire spinel and ghanospinel, Chlorspinel | |
| Chrysoberyl | 3.68–3.78 | 8.5 | Beryllium aluminum oxide | Chrysoberyl, Alexandrite, Cat's eye | |
| Topaz | 3.5–3.6 | 8 | Aluminum silicate fluoride hydroxide | none listed | |
| Diamond | 3.51 | 10 | Carbon | none listed | |
| Peridot | 3.22–3.45 | 7 | Magnesium iron silicate | none listed | |
| Jadeite | 3.1–3.5 | 6 | Sodium aluminum silicate | none listed | |
| Tourmaline | 3.03–3.25 | 7–7.5 | Complex aluminum borosilicate | Achorite, Brazilian emerald, Dravite, Indicolite, Rubellite, Siberite, Verdilite | |
| Nephrite | 2.9–3.1 | 6 | Calcium magnesium silicate | none listed | |
| Lapis lazuli | 2.7–2.9 | 5–5.5 | A rock composed mainly of the mineral lazurite with variable amounts of pyrite (brassy flecks) and white calcite | none listed | |
| Beryl | 2.63–2.91 | 7.5–8 | Beryllium aluminum silicate | Emerald, Aquamarine, Morganite, Heliodore, Red beryl, Goshenite | |
| Turquoise | 2.6–2.8 | 5–6 | Hydrous copper aluminum phosphate | none listed | |
| Feldspar | 2.55–2.76 | 6–6.5 | Two distinctly different alkali alumino silicates: the Plagioclase and the Alkali Feldspar Series | Labradorite, Sunstone, Peristerite, Amazonite, Moonstone | |
| Quartz | 2.65 | 7 | Silicon dioxide or silica | Rock crystal, Amethyst, Citrine, Morion, Smoky quartz or cairngorm, Rose quartz, Green quartz or praziolite, Agate | |
| Opal | 1.98–2.25 | 5.5–6.5 | Hydrated silica | White opal, Black opal, Water opal, Fire opal |
Other gemstones
Back to the full gemstone hardness and weight guide
Gemstones, Weighed
Seventeen gem minerals by specific gravity, drawn to scale, with their hardness on the Mohs scale.
The chart on this page, printed properly.