Why gemmologists measure four properties
Every gem species has a characteristic set of physical constants. No single number identifies a stone conclusively, but combining four measurements narrows the field rapidly, often to a single species. This reference covers Mohs hardness, refractive index (RI), specific gravity (SG), and crystal system for 50+ common and semi-precious gemstones.
What each property reveals
Mohs hardness ranks scratch resistance on a 1–10 scale. Each step represents the ability of the harder mineral to scratch the softer one. For jewellery, hardness above 7 resists the quartz dust in everyday grime; below 7 and surfaces can dull over time. But hardness and toughness differ: diamond scores 10 for hardness yet cleaves along its octahedral planes under a sharp blow, while nephrite jade sits around 6 but is extremely tough because of its interlocking fibrous structure.
Refractive index (RI) measures how much a stone bends light as it enters the surface. It is read with a refractometer and is one of the most reliable identifiers. Many stones show a range of RI values (birefringence) depending on the axis of the crystal through which the light travels; the table lists the typical range. High RI correlates with strong brilliance — diamond’s exceptional sparkle comes partly from its very high RI.
Specific gravity (SG) is the density of the stone relative to water. A stone with SG 4.0 weighs four times as much as the same volume of water. Gemmologists measure it by hydrostatic weighing (weighing in air, then weighing suspended in water, and applying Archimedes’ principle) or by heavy-liquid flotation. SG distinguishes stones of similar RI — for example, colourless topaz (SG 3.53) from colourless beryl (SG 2.72), both of which could appear similar to the eye.
Crystal system describes the internal atomic symmetry: cubic (isometric), hexagonal, trigonal, tetragonal, orthorhombic, monoclinic, triclinic, or amorphous. Stones in the cubic system are singly refractive (one RI value); all others are doubly refractive and show birefringence. The crystal system also determines cleavage directions and optical properties visible under a loupe.
Using this table to narrow an identification
The search box filters every column simultaneously. A practical approach when identifying an unknown stone:
- Measure what you can — hardness with test sticks, RI on a refractometer, SG by weighing.
- Search or filter the table by the measured RI range.
- Cross-check hardness and SG to eliminate candidates with matching RI but different density.
- Use the crystal system and any observed optical effects (double refraction, pleochroism) to confirm.
Example
A colourless stone reads RI approximately 1.62, hardness around 8, and SG around 3.53–3.57. Filtering for RI near 1.62 and hardness 8 points strongly to topaz. Diamond (RI 2.42, SG 3.52) is ruled out by RI; quartz (RI 1.54–1.55, SG 2.65) by both RI and SG. No single property is conclusive — always combine at least three measurements and, for a valuable stone, refer to a qualified gemmologist.
Durability in jewellery settings
For practical purchasing decisions, hardness above 7 is the usual recommendation for rings worn daily (which encounter quartz grit at SG 2.65 hardness 7 in dust). Stones below this — fluorite (4), opal (5.5–6.5), moonstone (6–6.5) — are better suited to pendants or earrings with protective settings. Toughness, which relates to cleavage and fracture rather than Mohs hardness, is a separate consideration the table does not capture.