Reading RI, SG, and Mohs on a Gem Listing

Refractive index, specific gravity, and Mohs hardness are three measurements that appear in gem listings and mean something specific, so it is worth knowing exactly what each one measures and where its authority ends.

Refractive index

Refractive index is a measure of how much a stone bends light as the light enters it. It is expressed as a ratio: the speed of light in air divided by the speed of light inside the material. A refractometer reads it from a flat, polished surface in contact with a drop of high-index liquid.

• Single-refraction stones give one value. Spinel is about 1.718, and diamond is 2.417.
• Double-refraction stones give two values or a range, because light splits into two paths inside the crystal. Quartz reads about 1.544 to 1.553, beryl about 1.577 to 1.583, and corundum about 1.762 to 1.770.

What it proves: the stone's species is consistent with those numbers. Quartz, beryl, and corundum do not overlap in refractive index, so a reading can often rule out whole families of material at once.

Where it stops: the reading needs a flat polished surface, so a cabochon usually cannot be measured. Very high-index stones such as diamond exceed the range of a standard refractometer and need other methods. And a reading narrows species without settling origin, growth method, or treatment.

Specific gravity

Specific gravity compares a stone's density to the density of water. The stone is weighed in air, then weighed again suspended in water; the first weight divided by the difference between the two gives the figure.

Some useful anchors: quartz is about 2.65, beryl about 2.70 to 2.90, tourmaline about 3.00 to 3.20, corundum about 4.00, zircon about 4.60 to 4.70, and hematite about 5.20.

What it proves: it separates species that look alike. A red stone weighing in near 4.00 points toward corundum, while one near 3.60 points toward spinel. Measured alongside refractive index, it becomes a strong filter.

Where it stops: it requires a loose stone. Once a stone is set, it cannot be weighed hydrostatically, and a listing that quotes specific gravity for a mounted stone is quoting something measured earlier or estimating it. Overlapping ranges mean a single figure rarely identifies a species on its own.

Mohs hardness

Mohs hardness measures resistance to scratching on a comparative scale from 1 (talc) to 10 (diamond). Quartz sits at 7, topaz at 8, corundum at 9, and diamond at 10. The scale is not proportional: diamond is far harder than corundum, even though the numbers are one apart.

What it proves: it tells you how a stone behaves in daily wear, which is a practical question rather than an identification one. It says nothing about toughness, which is resistance to breaking or chipping. Diamond is the hardest mineral and still has planes along which it splits cleanly.

What the three together can and cannot establish

Three measurements taken together rule out a great deal of material. Glass usually reads near 1.5 with a single value and a specific gravity around 2.5, so it fails most comparisons immediately. Cubic zirconia reads about 2.16 with a specific gravity near 5.8, which separates it from diamond.

They do not do everything.

• They cannot reliably separate natural from lab-grown in every species. Synthetic sapphire has essentially the same refractive index and specific gravity as the natural material.
• They cannot establish origin. Where a stone formed is an opinion given by a gem laboratory using instruments beyond these three.
• They cannot catch every treatment. Oiling, dyeing, and glass filling often need magnification or spectroscopy to see.

A listing that states measurements is telling you the stone's identity is consistent with the species named, and that the seller is willing to be specific. That is worth something real and is not the same as an independent laboratory report, which is what settles growth method, origin, and treatment when the money is serious.

Ask for an independent laboratory report on any stone where those questions change the price, and treat stated measurements as the foundation rather than the verdict.