Specific gravity (SG) is the ratio of a gemstone's mass to the mass of an equal volume of water. It is a fixed physical constant — like refractive index, it does not change with color, cut, lighting condition, or most treatments. (Surface coatings such as wax, resin, or oil create a small downward bias; this is noted below.) Combined with RI, SG identifies most gemstones to species level without any further testing.

The practical value becomes clear with species that share overlapping RI ranges. A red spinel and a ruby can produce similar refractometer readings in the 1.76x range. Specific gravity immediately separates them: ruby (corundum) runs 3.97–4.05, while spinel runs 3.52–3.57 — a gap of 0.4 or more. This is well within what a simple hydrostatic weighing setup can resolve, even with a consumer jeweler's scale.

The Hydrostatic Weighing Formula

Archimedes' principle states that a submerged object displaces its own volume of water. The weight lost in water equals the weight of water displaced. SG is simply the ratio:

SG = Weight in air Weight in air − Weight in water

Worked example:
Stone weighs 2.50 ct in air · Weighs 1.86 ct suspended in water
SG = 2.50 ÷ (2.50 − 1.86) = 2.50 ÷ 0.64 = 3.91
Consistent with ruby (3.97–4.05) or zircon low end — RI would resolve; at 3.91 this is more likely a rhodolite garnet.

The denominator (W_air − W_water) is the buoyancy force — the weight of water displaced by the stone. The smaller this number, the lighter the stone relative to water, meaning lower SG. A stone with SG exactly 1.00 would weigh zero in water.

What You Need

1

Precision balance accurate to 0.01 ct (0.002g) or better. A jeweler's digital scale in this range costs $20–$60 and is entirely sufficient. A kitchen scale (1g resolution) is not adequate for stones under roughly 10 carats.

2

Beaker of distilled water at room temperature (~23°C). Distilled water has a consistent density. Tap water with high mineral content introduces a small but measurable error. Distilled water is inexpensive and the standard for accurate work.

3

Hydrostatic weighing bridge. A metal frame that spans the scale pan and supports a beaker of water above the pan. Commercial versions are available from gem supply houses (Kassoy, Stuller). A DIY version works: suspend the stone over the beaker using wire looped through the scale's hook or pan edge, or use a retort stand and wire to hold the stone in the water while the wire connects down to the scale.

4

Fine wire or synthetic thread. 28–32 AWG copper wire or fine nylon thread. Must be thin enough that its own displacement in water is negligible. This is tared out before the measurement anyway, but thinner wire introduces less error and is easier to work with small stones.

5

Forceps or stone holder. To place and position the stone without contaminating it with skin oils, which can trap a small air layer and reduce the apparent SG reading.

Step-by-Step Measurement

1

Weigh the stone in air (W_air)

Clean the stone with a lint-free cloth to remove oils. Set it directly on the scale pan. Record the weight in carats or grams to the full precision of your scale. This is W_air. Do not round — you will subtract from this value, so rounding errors compound.

2

Set up the hydrostatic bridge

Position the bridge over your scale so the beaker of water sits above the scale pan and the scale can still register weight. The stone will hang from wire through the water and ultimately connect to the scale. Commercial bridges usually clamp to the scale body or the bench surface with the beaker elevated above the scale pan.

3

Suspend the stone in water

Loop wire around the stone's girdle, or use a small wire basket for round or irregular stones. Lower the stone into the beaker of distilled water, ensuring it is fully submerged and not touching the beaker walls or bottom. Any contact will transfer force to the beaker rather than the scale, producing a falsely low reading. Check that no air bubbles cling to the stone surface — they add buoyancy and lower the apparent SG. Brush gently with a fine brush if needed.

4

Tare the scale, then weigh in water (W_water)

With the bridge, beaker, and wire in place but the stone removed, press TARE (zero) on your scale. Now attach the stone and lower it fully into the water. The scale now reads only the apparent weight of the stone in water. Record this value as W_water. It will be less than W_air — that difference is the buoyancy.

5

Calculate SG

SG = W_air ÷ (W_air − W_water). If W_air = 2.50 ct and W_water = 1.86 ct, then SG = 2.50 ÷ 0.64 = 3.91. A result ending in more than two significant decimal places is false precision for most bench setups — report to two decimal places unless your scale resolution warrants more.

6

Cross-reference the result

Enter your SG value in GemID alongside your RI reading. The engine filters all 130 gem species against both values simultaneously. Alternatively, consult the SG table below. If your result falls at the boundary of two species' ranges, look at the optic character from your refractometer reading — isometric (SR) vs. non-isometric (DR) often resolves the ambiguity immediately.

Accuracy Tips

Use distilled water

Hard tap water (high mineral content) is measurably denser than distilled. Use distilled water at room temperature (~23°C) for consistent results.

Eliminate air bubbles

Air bubbles clinging to the stone's surface add buoyancy and reduce the apparent SG. Brush the stone gently with a fine watercolor brush while submerged.

Tare out bridge and wire

Tare the scale with everything except the stone in place. Recalibrate after any shift in the bridge position or wire geometry.

Small stones have poor accuracy

Stones under 0.1 ct produce SG readings with ±0.15 or greater uncertainty. For small stones, rely primarily on RI and optic character.

Watch for surface treatments

Wax, resin, oil fracture filling, and polymer impregnation all lower the apparent SG. A treated emerald may read closer to 2.60 than 2.72.

No touching the beaker

The stone must not touch the beaker walls or bottom. Contact transfers buoyancy force to the beaker and lowers the scale reading.

Specific Gravity Reference — 20 Gem Species

These ranges reflect natural, untreated specimens. Synthetic counterparts typically fall within the same range unless noted. Surface coatings reduce measured SG; heavily included stones (e.g., emerald with extensive fracture filling) may read low.

Gem Species	SG Range	Crystal System	Common Confusion
Ruby (corundum)	3.97–4.05	Trigonal	Spinel (3.52–3.57), garnet
Sapphire (corundum)	3.97–4.05	Trigonal	Tanzanite (3.35), synthetic sapphire (same SG)
Spinel	3.52–3.57	Isometric	Ruby — SG gap of ~0.45 immediately separates
Tsavorite Garnet	3.57–3.73	Isometric	Demantoid (3.82–3.88), spinel
Pyrope Garnet	3.65–3.87	Isometric	Almandine — SG overlaps at high pyrope/low almandine
Almandine Garnet	3.95–4.20	Isometric	Ruby — RI (garnet SR, corundum DR) separates immediately
Rhodolite Garnet	3.74–3.94	Isometric	Pyrope/almandine blend — intermediate SG
Emerald (beryl)	2.67–2.90	Hexagonal	Treated emerald reads low (~2.60); green glass reads ~2.50
Aquamarine (beryl)	2.67–2.80	Hexagonal	Same species as emerald; RI and SG overlap
Tanzanite (zoisite)	3.35	Orthorhombic	Strong trichroism and DR are the confirming tests
Tourmaline	2.90–3.26	Trigonal	Wide SG range due to compositional variation; RI required
Amethyst / Quartz	2.65	Trigonal	Consistent SG regardless of color; very low for a gem
Topaz	3.49–3.57	Orthorhombic	Overlaps spinel SG; RI and biaxial optic character confirm
Alexandrite (chrysoberyl)	3.71–3.75	Orthorhombic	Color change is visual; SG + RI are the physical confirmation
Diamond	3.52	Isometric	RI 2.42 (off-scale); thermal conductivity probe standard test
Moissanite	3.22	Hexagonal	SG vs. diamond (3.52) clearly separates; DR vs. diamond SR
Cubic Zirconia (CZ)	5.60–5.95	Isometric (synthetic)	SG far above all natural gems — unmistakable by weight alone
Zircon	3.90–4.73	Tetragonal	Wide range due to metamictization; low-type zircon reads ~3.90
Peridot	3.28–3.48	Orthorhombic	Strong birefringence (back-facet doubling) is the visual tell
Jadeite	3.20–3.36	Monoclinic	Nephrite: 2.90–3.10; jadeite SG higher; RI confirms species

Heavy liquids: Methylene iodide (SG ~3.32), bromoform (SG ~2.89), and Clerici solution (SG up to 4.25) are historically used to bracket SG by float/sink behavior. They are toxic, regulated, and require disposal as hazardous waste. Hydrostatic weighing gives the same result safely and is the standard bench method for a reason. Avoid heavy liquids unless you have a fume hood and proper disposal infrastructure.

When SG Alone Resolves the Identification

In four situations, specific gravity measurement alone closes the identification without needing the refractometer:

Diamond vs. Moissanite

Diamond SG 3.52, moissanite SG 3.22. The gap is 0.30 — well within the precision of any decent bench scale on a stone of 0.5 ct or larger. Thermal conductivity testers are the standard, but SG provides a secondary confirming measurement.

Decisive

Cubic Zirconia vs. Any Natural Gem

CZ's SG of 5.60–5.95 is far above every natural gem species. A 1-carat CZ feels noticeably heavier than a 1-carat diamond. Even without a scale, experienced dealers detect CZ by heft. On a scale, it is unmistakable.

Decisive

Jadeite vs. Nephrite

Jadeite SG 3.20–3.36, nephrite SG 2.90–3.10. Some overlap exists at 3.20–3.10 boundary. Use RI to confirm (jadeite 1.654–1.667, nephrite 1.606–1.632). SG alone narrows it; RI closes it.

Partial overlap — use RI

Synthetic Corundum vs. Natural

Synthetic corundum has SG 3.97–4.00, natural corundum 3.97–4.05 — essentially the same. SG cannot distinguish synthetic from natural corundum. This requires inclusion observation (microscopy) or advanced spectroscopy.

SG cannot separate these

Frequently Asked Questions

My stone is in a setting — can I still measure SG?

No. A mounted stone cannot be accurately measured by the hydrostatic method. The setting metal adds mass in air but also displaces water, creating errors in both W_air and W_water. The resulting SG will be meaningless. For stones in settings, use other diagnostics: refractometer (if a flat facet is accessible), fluorescence, spectroscopy, or absorption characteristics. Unmounting is the only reliable path to an accurate SG measurement.

How accurate does my SG need to be?

Within ±0.05 is sufficient for most identifications. Ruby (3.97–4.05) and spinel (3.52–3.57) are separated by roughly 0.40 — you need much less precision than that. Some species pairs have tighter SG ranges that genuinely overlap (see the identify cards above), and those require RI as the second axis. For most gemological decisions, two decimal places from a 0.01g-resolution scale on a stone above 0.5 ct is adequate.

Can I use a kitchen scale?

A typical kitchen scale reads to 1 gram (5 carats). For stones under about 10 carats, this resolution is insufficient — the buoyancy reading (W_air − W_water) may be less than your scale's resolution, making the calculated SG meaningless. Use a jeweler's digital scale accurate to at least 0.01g (0.05 ct). These are inexpensive ($20–$60) and adequate for all bench work above 0.1 ct.

What if my reading does not match any gem in the table?

An SG significantly outside all expected ranges suggests one of four situations: (1) An imitation or simulant — glass runs 2.30–4.20 depending on formulation; synthetic materials can be almost any density. (2) A composite stone — doublets and triplets combine materials of different densities; the bulk SG reflects a blend. (3) A coated stone — resin or polymer impregnation lowers apparent SG, sometimes substantially (treated emerald, polymer-filled turquoise). (4) Measurement error — verify that the stone was fully submerged, not touching the beaker, and free of air bubbles, and that the wire and bridge were properly tared.

How to Measure Specific Gravity of Gemstones