Specific Gravity Tester

Calculate specific gravity from weight in air and weight in water. Match results to known gemstones.

Calculator

How to Use

  1. 1
    Weigh the gem in air first

    Place the clean, dry gemstone on a precision scale accurate to at least 0.01 gram and record the weight in air (Wₐᴵᴿ). Ensure the scale is calibrated and that the gem is free from oil, coatings, or moisture that could affect the measurement.

  2. 2
    Weigh the gem fully submerged in water

    Suspend the gem fully submerged in distilled water at room temperature (approximately 23°C), using a fine wire or gem holder, and record the submerged weight (Wₕ₂ₒ). The difference between air and water weights represents the buoyancy force equal to the weight of water displaced.

  3. 3
    Calculate specific gravity and identify the gem

    Specific gravity = Wₐᴵᴿ ÷ (Wₐᴵᴿ − Wₕ₂ₒ). Compare the result to the tool's reference table of known gem SG values. Diamond measures 3.52, corundum 3.97–4.05, and quartz 2.65; SG is one of the most reliable identification indicators when combined with refractive index measurement.

About

Specific gravity testing is one of the oldest and most reliable gemological identification methods, based on Archimedes' principle that a submerged object displaces fluid equal to its own volume. Since different mineral species have characteristic densities determined by their atomic composition and crystal structure, SG measurement provides direct chemical information about the gem material. The method requires only a precision scale and a vessel of water, making it accessible in any gemological laboratory or well-equipped jewelry store.

The hydrostatic weighing method—comparing a gem's weight in air to its weight fully suspended in water—provides the calculation: SG = (weight in air) / (weight in air − weight in water). The difference between the two weights equals the buoyancy force, which by Archimedes' principle equals the weight of water displaced, which for water of density 1.0 g/cm³ equals the volume of the stone in cubic centimeters. This calculation requires no additional information about stone shape or dimensions.

Modern gemological laboratories use heavy liquids of known specific gravity as an alternative screening method: a gem will float in liquid of lower SG than itself, sink in liquid of higher SG, and remain suspended in liquid of equal SG. Bromoform (SG 2.89), methylene iodide (SG 3.33), and Clerici solution (SG 4.15) were historically used but are being replaced with less toxic alternatives such as sodium polytungstate solutions and fluorine-based compounds. Heavy liquid testing allows rapid screening of many stones but provides less precise values than hydrostatic weighing. The combination of SG with refractive index measurement provides sufficient data to identify most gem species without advanced spectroscopic equipment.

FAQ

Why is specific gravity measured in water at a specific temperature?
Water's density changes with temperature: pure water is densest at 3.98°C (density = 1.000 g/cm³) and becomes slightly less dense at higher temperatures. At 23°C, water density is approximately 0.9976 g/cm³. For most gemological purposes this difference is negligible, but precision testing uses distilled water at a specified temperature. Distilled water is preferred over tap water to avoid dissolved minerals that slightly increase density. Some advanced gemological setups correct for water density at the measured temperature, while routine testing accepts the small error introduced by room-temperature measurements.
What specific gravity values indicate ruby versus garnet?
Ruby (corundum) has a specific gravity of 3.97–4.05, while red garnets vary by species: pyrope garnet is 3.65–3.87, almandine garnet is 3.95–4.20, and spessartine garnet is 4.12–4.20. The SG values overlap somewhat, making them insufficient alone to distinguish ruby from some garnets, but when combined with refractive index measurement the distinction becomes clear: ruby has RI 1.762–1.770, while garnets are singly refractive (no double refraction) with characteristic RI values. This combination test is a standard gemological diagnostic procedure.
Can specific gravity testing damage a gemstone?
Standard hydrostatic weighing in distilled water does not damage most gems. However, several exceptions exist: emeralds are typically oiled (filled with cedar oil or resins) to improve apparent clarity, and water immersion may affect these fillings over time, though brief testing rarely causes problems. Pearls and coral are organic materials that can absorb water and should not be submerged. Lapis lazuli, malachite, turquoise, and other porous gems can absorb water, potentially affecting future appearance. Doublets and triplets (composite stones with adhesive layers) may separate with water exposure. For these sensitive materials, other identification tests are preferred.
How precise does the scale need to be for accurate SG measurement?
The precision required depends on stone size. For gems above 5 carats (1 gram), a scale accurate to 0.01 gram provides acceptable SG precision of approximately ±0.02 SG units, sufficient to distinguish most gem species. For small stones (under 1 carat, under 0.2 grams), errors of 0.01 gram become proportionally large, reducing accuracy to perhaps ±0.1–0.2 SG units. Professional gemologists prefer scales accurate to 0.001 gram for reliable small-stone SG measurement. The GIA gemology program recommends hydrostatic balance setups capable of measuring to four significant figures for research applications.
What gems have unusually high or low specific gravity?
Among well-known gem materials, hematite has notably high SG at 4.95–5.16, giving it a distinctly heavy feel in the hand. Zircon (3.90–4.73 depending on metamict alteration state) is also denser than most gems. At the other extreme, amber has exceptionally low SG at 1.08–1.10—barely denser than water—so it floats in saturated salt water, a traditional test for amber authenticity. Amber can be distinguished from glass or plastic imitations by this float test. Opal (2.15–2.23) and most feldspars (moonstone, labradorite at approximately 2.56–2.62) have low SG values that feel light compared to corundum or garnet of the same size.
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