Gemstone Specific Gravity Chart

The chart

Every value below is the default our calculators use, with the typical range we use for estimates. Values marked approximate are ones where published sources disagree or the material varies a lot. The last column is the size of a 1 ct round stone of that material, generated by our engine at typical depth.

What specific gravity means for size

Specific gravity is how heavy a material is compared with water. If a stone has SG 4.00, a piece of it weighs four times as much as the same volume of water. For shoppers, SG explains why carats and millimetres never line up the same way twice. Two stones with the same weight but different SG have different volumes, so the lighter material looks larger.

Because weight grows with volume, and volume with the cube of size, the effect on diameter is smaller than the effect on weight. Compare cubic zirconia (cz) and opal: their SG differs by a factor of about 2.7, but at 1 ct the round opal is only about 1.39 times the diameter of the cubic zirconia.

How SG is measured

The classic method is hydrostatic weighing. Weigh the loose stone in air, then weigh it again hanging in water on a fine wire. The stone weighs less in water by exactly the weight of the water it displaces. SG is the weight in air divided by that difference. Gemologists also use heavy liquids of known density to see whether a stone floats or sinks, but those liquids are toxic and not for home use.

If you have a measured SG, enter it in the "More options" panel of the mm to carat calculator. It replaces the typical value and narrows the range.

Gem by gem notes

A short practical note for each material we support. Sizes are 1 ct round stones at typical depth.

Opal

SG 2.15 (range 1.98 to 2.25), Mohs 5.5 to 6.5. A 1 ct round is about 7.5 mm. Solid opal and opal doublets or triplets differ in weight. Ask which you are buying.

Amethyst (quartz)

SG 2.65 (range 2.63 to 2.66), Mohs 7. A 1 ct round is about 7.0 mm. Store away from strong sunlight, which can fade some amethyst.

Citrine (quartz)

SG 2.65 (range 2.63 to 2.66), Mohs 7. A 1 ct round is about 7.0 mm. Much citrine on the market is heated amethyst. That is common and normally fine, but should be disclosed.

Quartz (smoky, rose, clear)

SG 2.65 (range 2.63 to 2.66), Mohs 7. A 1 ct round is about 7.0 mm. Rose quartz is often cloudy and cut as cabochons. Use the cabochon setting in the calculator.

Lab-grown emerald

SG 2.67 (range 2.65 to 2.70, approximate), Mohs 7.5 to 8. A 1 ct round is about 7.0 mm. Ask which growth method was used if you want a precise weight estimate, since flux and hydrothermal material differ slightly in density.

Aquamarine

SG 2.69 (range 2.66 to 2.74), Mohs 7.5 to 8. A 1 ct round is about 7.0 mm. Paler stones need more size to show color. Compare millimetres when choosing between two pale stones.

Emerald

SG 2.72 (range 2.67 to 2.78), Mohs 7.5 to 8. A 1 ct round is about 6.9 mm. Avoid ultrasonic cleaners. They can damage fillers and fractures that most emeralds have.

Turquoise

SG 2.76 (range 2.60 to 2.90, approximate), Mohs 5 to 6. A 1 ct round is about 6.9 mm. Much turquoise is stabilised with resin, which changes its density. Ask about treatment.

Morganite

SG 2.80 (range 2.71 to 2.90, approximate), Mohs 7.5 to 8. A 1 ct round is about 6.9 mm. Pink to peach color can fade slightly or shift with light. Look at the stone in daylight if you can.

Lapis lazuli

SG 2.80 (range 2.70 to 2.90, approximate), Mohs 5 to 5.5. A 1 ct round is about 6.9 mm. Lapis is a rock, so pyrite and calcite content change its density from piece to piece.

Tourmaline

SG 3.06 (range 3.00 to 3.26), Mohs 7 to 7.5. A 1 ct round is about 6.7 mm. Many tourmalines show different color shades from different directions. Look at the face-up color, not the side view.

Moissanite (lab-created)

SG 3.22 (range 3.17 to 3.22), Mohs 9.25. A 1 ct round is about 6.6 mm. Check whether the weight is DEW or real. Most loose moissanite is also described by millimetre, which is the clearest figure.

Peridot

SG 3.34 (range 3.27 to 3.37), Mohs 6.5 to 7. A 1 ct round is about 6.5 mm. Peridot is softer than many gems. Protective settings help in rings.

Jadeite jade

SG 3.34 (range 3.30 to 3.38), Mohs 6.5 to 7. A 1 ct round is about 6.5 mm. Jadeite and nephrite are both called jade but differ in density. Ask which one you have.

Tanzanite (zoisite)

SG 3.35 (range 3.28 to 3.38), Mohs 6 to 7. A 1 ct round is about 6.5 mm. Tanzanite is relatively soft and can be brittle. Pendants and earrings are safer than everyday rings.

Diamond (natural)

SG 3.52 (range 3.50 to 3.53), Mohs 10. A 1 ct round is about 6.5 mm. Ask for a grading report with measurements on any important diamond. The measurements tell you more about face-up size than the weight does.

Lab-grown diamond

SG 3.52 (range 3.50 to 3.53), Mohs 10. A 1 ct round is about 6.5 mm. Grading reports for lab-grown stones list the same measurements as natural ones, so compare them the same way.

Topaz

SG 3.53 (range 3.49 to 3.57), Mohs 8. A 1 ct round is about 6.4 mm. Avoid sharp knocks. Topaz cleaves easily along one direction despite its hardness.

Spinel

SG 3.60 (range 3.57 to 3.70), Mohs 8. A 1 ct round is about 6.3 mm. Measure and use a custom SG if you can, because some spinels are noticeably denser than average.

Garnet, tsavorite

SG 3.61 (range 3.57 to 3.65, approximate), Mohs 7 to 7.5. A 1 ct round is about 6.3 mm. Published densities vary. If weight matters, measure SG and use the custom field.

Garnet, pyrope

SG 3.78 (range 3.62 to 3.87, approximate), Mohs 7 to 7.5. A 1 ct round is about 6.2 mm. Pyrope is often the bright red garnet in antique clusters, usually in small sizes.

Garnet, rhodolite

SG 3.84 (range 3.74 to 3.94, approximate), Mohs 7 to 7.5. A 1 ct round is about 6.2 mm. Rhodolite is a pyrope and almandine mix, so its density sits between the two.

Ruby

SG 4.00 (range 3.95 to 4.05), Mohs 9. A 1 ct round is about 6.1 mm. Ask about fillers in fractures. Lead glass filling lowers durability and must be disclosed.

Sapphire

SG 4.00 (range 3.95 to 4.05), Mohs 9. A 1 ct round is about 6.1 mm. Ask for face-up measurements as well as weight, because deep pavilions are common.

Lab-grown ruby or sapphire

SG 4.00 (range 3.95 to 4.05), Mohs 9. A 1 ct round is about 6.1 mm. Lab-grown ruby and sapphire are chemically corundum, so a size chart for natural stones applies.

Garnet, almandine

SG 4.05 (range 3.93 to 4.30), Mohs 7 to 7.5. A 1 ct round is about 6.1 mm. Small dark garnets can look black. Choose a lighter garnet or a larger size for brightness.

Garnet, spessartine

SG 4.15 (range 4.12 to 4.20, approximate), Mohs 7 to 7.5. A 1 ct round is about 6.0 mm. Orange spessartine is one of the denser garnets, so it faces up a little small for its weight.

Zircon

SG 4.35 (range 3.93 to 4.73, approximate), Mohs 6 to 7.5. A 1 ct round is about 5.9 mm. Natural zircon varies a lot in density. Do not confuse it with cubic zirconia, which is a lab-made simulant.

Cubic zirconia (CZ)

SG 5.80 (range 5.60 to 6.00), Mohs 8 to 8.5. A 1 ct round is about 5.4 mm. Buy by millimetre. A CZ weight figure can mean real weight or diamond equivalent, and the two differ a lot.

Sources for these values

The values in this chart were cross-checked against 6 references, chiefly the International Gem Society density tables, the USGS Natural Gemstones report and GemSelect. The full list, with the exact pages consulted, is on the How we calculate page. Each material's sources are recorded in the site's data file and conflicts are documented on the How we calculate page. To compare materials directly, use the DEW and species comparison tool.