Gemology — Physical Properties

Physical Properties

Hardness, cleavage, and fracture — the hands-on properties gemologists test every day. Interactive Mohs scale, scratch test simulator, and 3D cleavage explorer.

Developed by Akila Amarathunga · Gemélia & Co.

Section 1

The Mohs Scale of Hardness

In 1812, Friedrich Mohs arranged ten minerals in order of their ability to scratch one another. Each mineral on the scale can scratch all those below it and is scratched by all those above. This gives us an ordinal ranking — not an absolute measurement.

Key concept: Hardness is the resistance of a smooth surface to scratching (not to breaking or chipping — that's toughness). A mineral of hardness 7 will scratch one of 6, but the difference in absolute hardness between 9 and 10 is vastly greater than between 1 and 9.

Click any mineral to learn more. The bar shows absolute hardness (Vickers scale) — notice how diamond dwarfs everything else.

Section 2

Scratch Test Simulator

The scratch test is the gemologist's quick field test for hardness. Choose a testing tool and a specimen to see the result. In real life, you would draw the tool firmly across the specimen and check if it leaves a scratch.

Important: Never scratch-test a finished gemstone — this damages the surface. Use rough material or inconspicuous edges. Also check the result isn't just a powder mark (wipe it off and check for an actual groove).
Select a tool and specimen, then click the surface to test
Section 3

Absolute vs Relative Hardness

Mohs' scale is ordinal — it tells you the order, not the magnitude of difference. The actual (absolute) hardness, measured by the Vickers indentation test, reveals a very different picture.

The diamond gap: Diamond (Mohs 10) has a Vickers hardness of ~10,000. Corundum (Mohs 9) is only ~2,000. That means diamond is roughly five times harder than the next hardest mineral — yet Mohs' scale places them just one step apart.

This chart shows both scales side by side. Hover over each bar to compare.

Section 4

Directional Hardness

Hardness isn't always the same in every direction. Because crystal structures have different atomic densities along different axes, some minerals are significantly harder in one direction than another.

Kyanite is the classic example: hardness ~4.5 parallel to the crystal's length (c-axis) but ~6.5 across it. This two-hardness property gives kyanite its name — from the Greek kyanos (blue) — and makes it tricky to cut.

Diamond also shows directional hardness. It is slightly softer parallel to octahedral faces — which is precisely why diamond cutters can shape diamond with diamond powder applied in the correct direction.

Kyanite

H ≈ 4.5 along length · H ≈ 6.5 across

Diamond

Hardest ⊥ cube faces · softest ∥ octahedral faces

Section 5

Hardness in Practice

Why does hardness matter? Because everyday dust contains quartz particles (hardness 7). Any gemstone worn in a ring must resist being scratched by the fine quartz in dust that settles on every surface. This is why gemologists consider 7 the minimum hardness for ring stones.

The Rule of 7:
🟢 7 and above — Safe for rings and daily wear. Diamond, ruby, sapphire, emerald, aquamarine, topaz, quartz, spinel, chrysoberyl.
🟡 5 to 6.5 — Wear with care. Best in earrings, pendants, brooches. Opal, turquoise, lapis lazuli, moonstone, apatite.
🔴 Below 5 — Cabinet stones or very occasional wear. Fluorite, calcite, pearl, amber.
📝 Knowledge Check — Hardness
1. Can quartz (Mohs 7) scratch feldspar (Mohs 6)?
2. What is the minimum Mohs hardness recommended for ring stones?
3. Diamond (Mohs 10) vs corundum (Mohs 9) — how many times harder is diamond in absolute terms?
Score: 0 / 3
Section 6

Cleavage

Cleavage is the tendency of a crystalline mineral to break along flat planes that are parallel to planes of atoms in its crystal structure. These are directions where the atomic bonds are weakest.

Key concept: Cleavage always follows the crystal structure — it is repeatable, consistent, and produces flat, often reflective surfaces. A mineral either has cleavage in a particular direction or it doesn't. Cleavage is described by its direction (which crystal plane), quality (perfect, good, poor), and number of directions.

Select a mineral to see its cleavage planes in 3D. Drag to rotate. Click Split to animate the crystal breaking along its cleavage plane.

Diamond — Octahedral cleavage (perfect) · 4 directions
Diamond cleaves parallel to the octahedron faces. This is how rough diamonds are shaped before cutting. Despite being the hardest mineral, diamond splits easily along these planes because the carbon bonds in the octahedral direction are slightly weaker.

Cleavage quality is described as:

Perfect — produces smooth, flat, reflective surfaces (mica, topaz, diamond)
Good / Distinct — clean but may show slight irregularities (feldspar)
Poor / Indistinct — rough, difficult to produce (beryl, quartz has none)

Why cleavage matters to gemologists: A gem with perfect cleavage (like topaz) can split during setting if the jeweller strikes it in the wrong direction. Emerald has poor cleavage but is brittle due to inclusions — a different problem entirely. Understanding cleavage helps with both identification and handling.
Section 7

Cleavage vs Parting vs Fracture

When a mineral breaks, it does so in one of three ways. Understanding the difference is essential for identification — the way a gem breaks tells you about its internal structure.

Cleavage

Breaks along atomic planes in the crystal structure. Repeatable, flat surfaces. Always in the same direction(s) for a given mineral.

Parting

Breaks along twin planes or zones of structural weakness. Looks like cleavage but is not repeatable — only occurs where twins exist.

Fracture

Breaks with no relation to crystal structure. Irregular surfaces. Every mineral fractures; not every mineral cleaves.

Parting vs Cleavage: Corundum (ruby/sapphire) shows parting along twin planes — often mistaken for cleavage. The key difference: cleavage occurs in every specimen of a mineral, but parting only occurs in twinned specimens. If you can find a specimen without the break direction, it's parting, not cleavage.
Section 8

Types of Fracture

When a mineral breaks in a direction that doesn't follow cleavage planes, the surface shape is called fracture. The type of fracture is a useful identification clue.

Conchoidal fracture is the most important for gemologists. Glass, quartz, obsidian, and opal all show conchoidal fracture — the curved, shell-like surface with concentric ripple marks. It's one of the quickest ways to distinguish glass from crystalline gems (which may show cleavage instead).
📝 Knowledge Check — Cleavage & Fracture
1. What type of break follows atomic planes in the crystal structure?
2. Conchoidal fracture is most useful for identifying…
3. Corundum shows parting, not cleavage. Why?
Score: 0 / 3
Enjoying Gemélia Learn? This free resource is built and maintained by a gemology student. Your support helps keep it free and fund new interactive modules.
Visit Gemélia Shop