Mechanical Durability of Lab Corundum (Ruby), with Cubic Zirconia Comparison

Gemdrop® Research, Scientific investigation of lab gem materials.

• Experimental work and observations: India Stephens, Gem Researcher / Jeweller / Creator
• Scientific interpretation and research: AL Barnard, Gemdrop®

Abstract

Lab gems are often described in terms of hardness, but hardness alone does not determine how a gem will behave during jewellery manufacture, everyday wear or exposure to heat. This exploratory study compared the response of a lab ruby and a blue cubic zirconia (CZ) to workshop abrasives, common metal objects, dropping and severe impact.

Both specimens resisted contact with a steel-wire burnishing brush, carbon-steel file and the common metal objects tested. Both also survived a drop of approximately 5 ft (1.5 m) onto laminate flooring without visible damage. Abrasive rotary tools produced surface damage to both materials, although the CZ sustained substantially greater abrasion and material loss than the ruby under the more aggressive tests. In a direct scratch comparison, the ruby scratched the CZ, while no visible reciprocal scratch was produced on the ruby. Both specimens shattered after two strikes from a 16 oz hammer.

Fragments of the blue CZ created during destructive testing were subsequently exposed to a jewellery torch. Heating caused pronounced colour changes: a dark blue specimen became deep amber-brown, while a paler blue specimen developed a pale seafoam-to-olive green colour.

The observations illustrate three distinct material properties that are sometimes conflated in jewellery: hardness, tenacity and thermal colour stability. They also show why the behaviour of one laboratory-made gem cannot necessarily be inferred from another gem of similar appearance.

1. Introduction

A lab gem used in jewellery may encounter very different forms of stress during its lifetime. It can be rubbed against metal objects, exposed to abrasive workshop tools, dropped onto a hard surface, struck by an object or subjected to elevated temperatures during manufacture and repair.

These conditions do not test the same property.

Hardness describes resistance to scratching and abrasion. Tenacity describes resistance to breaking and deforming, e.g., chipping, cracking and fracturing. A material may be highly resistant to scratching yet still fracture under a sufficiently concentrated impact. Heat introduces another consideration: a gem can remain physically intact while undergoing changes in colour or other optical properties.

This Gemdrop® experiment began with a practical question:

How difficult is it to damage a lab gem under conditions relevant to jewellery making and everyday handling?

A red labruby and a blue lab cubic zirconia (CZ) were subjected to a sequence of increasingly aggressive tests. This is important because ruby and cubic zirconia are fundamentally different materials.

The destructive portion of the experiment also produced fragments of the blue CZ. These created an opportunity for observing what happens when coloured CZ is exposed directly to the intense heat of a jewellery torch.

Together, the two experiments provide a practical demonstration of how lab gems materials respond to abrasion, impact and heat.

2. Background

2.1 Lab Ruby

Ruby is the red variety of corundum, crystalline aluminium oxide (Al₂O₃). Corundum has a Mohs hardness of approximately 9, making it one of the hardest gem materials commonly used in jewellery (Klein and Dutrow, 2007).

Its high hardness gives ruby excellent resistance to scratching by many materials encountered in ordinary use. Hardness does not, however, make ruby indestructible. Abrasives approaching or exceeding its hardness can damage its surface, and sufficiently severe impact can cause fracture.

2.2 Cubic Zirconia

Cubic zirconia is crystalline zirconium dioxide (ZrO₂) stabilised in its cubic crystal form. Gem-quality CZ commonly has a Mohs hardness around 8, lower than that of corundum (Klein and Dutrow, 2007).

Although CZ can therefore perform well against many everyday objects, harder abrasives can scratch or abrade it more readily than ruby.

Coloured CZ is chemically more complex than the simple formula ZrO₂ suggests. Stabilising oxides are added to maintain the cubic crystal structure at room temperature, while colour may be produced by additional dopant elements and/or electronic defects and colour centres within the crystal lattice. The observed colour can therefore depend on factors including chemical composition, oxidation state, oxygen vacancies and other lattice defects. Some of these colour-producing mechanisms can be modified by heating, making coloured CZ particularly interesting in thermal experiments

The colour observed in CZ can therefore depend upon composition, oxidation state and defects within the crystal structure.

3. Methods

3.1 Specimens

Faceted lab gem specimens were examined:

1) red corundum (ruby)

2) blue cubic zirconia

Before testing, the blue CZ had one small pre-existing surface scratch. The ruby showed minor, very faint machine marks.

These pre-existing features were noted so they would not be mistaken for damage produced during the experiment.

3.2 Mechanical Test Design

Three rounds of testing were conducted. The tests were selected to represent three broad situations relevant to jewellery:

1. contact with mechanical polishing or workshop tools;

2. contact with common metallic objects carried in a handbag or pocket;

3. accidental dropping and severe external impact.

For Rounds 1 and 2, each implement was applied to the surface of the gem for approximately 10 seconds.

The experiment was observational rather than instrumented. Applied pressure, tool angle, rotary speed and contact force were not standardised quantitatively.

3.3 Round 1 — Workshop and Polishing Tools

The specimens were exposed to:

• a steel-wire burnishing brush;

• a carbon-steel file;

• a 400-grit polishing disc in a rotary tool;

• a white-stone burr, described as ceramic-bonded corundum or silicon carbide;

• a diamond burr in a rotary tool.

3.4 Round 2 — Common Metal Objects

The specimens were exposed to:

• a stainless-steel watch strap;

• stainless-steel nail scissors;

• alloy house keys.

3.5 Round 3 — Drop and Impact

Each specimen was:

• dropped from approximately 5 ft (1.5 m) onto laminate flooring;

• subsequently struck twice with a 16 oz hammer.

3.6 Direct Scratch Comparison

As an additional qualitative test, points of the ruby and CZ were applied directly against one another to determine whether either material produced a visible scratch on the other.

3.7 Thermal Test of Cubic Zirconia

Following the destructive hammer test, fragments of the blue CZ were available for further experimentation.

A fragment from the darker blue material was exposed directly to a jewellery torch. A second, initially paler blue CZ specimen was also heated.

These thermal tests were exploratory. Temperature, heating duration, cooling rate and atmospheric conditions were not instrumentally controlled.

4. Results

4.1 Mechanical Testing

4.2 Metallic Transfer on the CZ

Contact with the stainless-steel watch strap initially appeared to have marked the CZ.

Closer inspection suggested that the visible metallic sheen resulted from material being removed from the steel and deposited on the gem surface, rather than the CZ itself being scratched.

This is a useful practical distinction. A visible streak on a gemstone does not necessarily indicate that material has been removed from the gem.

4.3 Direct Ruby–CZ Scratch Test

When the two gems were applied directly against one another:

• the ruby was not visibly scratched by the CZ;

• the CZ was visibly scratched by the ruby.

4.4 Qualitative Surface Observation

During testing, the ruby was perceived as having a more “slippery” surface than the CZ.

Because surface friction was not measured, this is retained as a qualitative observation rather than treated as a measured material property.

4.5 Thermal Response of the Blue CZ

Heating produced obvious changes in colour.

The initially dark blue CZ lost its blue appearance and developed a deep amber-brown colour.

A second specimen that had initially been a much paler blue responded differently, developing a pale seafoam-to-olive green colour.

The two visually similar blue CZ materials therefore showed markedly different responses to intense heating.

5. Interpretation

5.1 Abrasion Resistance

The mechanical results are broadly consistent with the known relative hardness of corundum and cubic zirconia.

Corundum has a Mohs hardness of approximately 9, whereas CZ is typically around 8. The greater abrasion and material loss observed on the CZ during the more aggressive rotary-tool tests is therefore unsurprising.

The direct scratch test provides a particularly simple illustration of the same relationship: the harder ruby scratched the softer CZ, while the reverse was not visibly observed.

The result should not be interpreted as a quantitative measurement of hardness, but it is consistent with established gemological expectations.

5.2 Why the Common Metal Objects Caused Little Damage

The steel watch strap, nail scissors and house keys produced no visible scratching of either gem under the conditions tested.

This is also consistent with relative hardness. Ordinary metals are generally much softer than either corundum or CZ and are therefore more likely to suffer abrasion themselves than to scratch these gem materials. Steel, and hardened steel, is ~4 - 6 on Moh's scale, significantly less hard than ruby and CZ.

The metallic streak transferred from the steel watch strap onto the CZ is an especially useful example. What initially appeared to be damage to the gem was instead material transferred from the softer metal. This well-known phenomenon is commonly demonstrated in introductory geology labs worldwide using streak plates.

5.3 Hardness Is Not Tenacity

Both specimens survived the approximately 5 ft drop onto laminate flooring without visible damage.

Both also shattered when subjected to two deliberate blows from a 16 oz hammer.

These observations should not be taken to define either material's tenacity quantitatively. The force experienced during a drop or hammer strike depends upon geometry, orientation, contact point, support and many other factors.

They do, however, illustrate an important distinction.

A gemstone's resistance to scratching does not mean that it cannot break.

Moh's hardness and tenacity describe different properties.

5.4 Heat-Induced Colour Change in Cubic Zirconia

The heating observations introduced a second material property: thermal colour stability.

CZ has a very high melting temperature (~2700 °C), but a material does not need to melt for its colour to change. Colour is determined by the way a material absorbs and transmits visible light, and that behaviour can be affected by changes occurring at the atomic and electronic scale.

One relevant feature of stabilised zirconia is the presence of oxygen vacancies: locations within the crystal structure where an oxygen ion would otherwise be expected.

These defects form part of the material's broader defect chemistry and can interact with stabilising or colouring species. Heating may alter defect populations, oxidation states or the local electronic environment around colour-producing centres.

Changes of this kind can alter which wavelengths of visible light are absorbed.

Change the absorption spectrum, and the colour perceived by the eye may change with it.

The two CZ specimens did not respond identically. One changed from dark blue to amber-brown; the other changed from pale blue to a seafoam or olive green.

The precise chemical mechanism cannot be established from visual observation alone and there are many possible contributors. The important result is therefore not that heating blue CZ will always produce a particular new colour, but the opposite:

two apparently similar coloured CZ materials may respond very differently to heat because their underlying material chemistry may differ.

6. Practical Implications for Jewellery

The experiment offers several practical observations for jewellery manufacture, repair and wear.

Lab ruby showed the high abrasion resistance expected of corundum and resisted the ordinary metallic objects used in this study. However, aggressive abrasive tools still produced visible surface damage.

Direct contact between a polished gem surface and an abrasive rotary tool should therefore be avoided simply because a stone has a reputation for being “hard.”

The blue CZ also performed well against the common metal objects tested but showed substantially greater susceptibility to the more aggressive abrasives.

For both materials, surviving an accidental drop under one set of conditions should not be interpreted as evidence that the material is impact-proof.

The heating results introduce a separate consideration for coloured CZ. A stone may remain physically intact at a temperature well below its melting point while undergoing a substantial change in colour.

Consequently, the thermal behaviour of colourless CZ should not automatically be assumed to apply to coloured CZ, and the behaviour of one coloured CZ should not automatically be assumed to predict another.

For jewellery processes involving concentrated heat, the particular material chemistry matters.

7. An Unexpected Second Experiment

One of the most useful features of this investigation was not planned.

The original experiment asked how difficult it was to damage a laboratory-made gem. The final impact test broke the CZ and created fragments. Those fragments provided material for another experiment. Heating then produced unexpected colour changes, which raised a new question about the chemistry responsible for the colour.

The pale-blue specimen produced perhaps the most appealing result. After heating, its newly developed pale green colour was considered attractive enough that it was retained rather than subjected to further destructive testing.

It may eventually become a pendant.

What began as a destructive experiment therefore produced an unexpected new gem, and a new research question.

This progression is characteristic of exploratory science: an observation that does not fit the original question can become the starting point for the next investigation.

8. Limitations

This study was designed as an exploratory practical experiment rather than a standardised materials test.

Only individual specimens were examined, and the results should not be treated as representative performance specifications for all lab ruby or cubic zirconia.

During mechanical testing, variables including applied pressure, contact angle, rotary-tool speed and exact abrasive composition were used at typical operating speeds. Damage was visually assessed. For the drop and hammer tests, the specimen was subjected to direct impact at the point of contact. The thermal experiment was opportunistic. Specimen temperature, torch distance, heating duration, atmosphere and cooling rate were not precisely recorded or controlled. The chemical composition of the coloured CZ specimens is under investigationand we hope in future to link the colour responses to particular dopants or defects.

The findings should be interpreted as observations of the particular specimens under the conditions tested, rather than universal claims extrapolated from the two materials tested.

9. Further Research

The experiment suggests several routes for more research.

Future Gemdrop® studies could:

• repeat abrasion tests using more materials;

• conduct testing with varied torch-heating times and approximate temperatures;
  

• heat multiple coloured CZ specimens under identical conditions;

• bust heat-changed gems and examine fresh fracture surfaces to determine whether colour change iextends through the material or is concentrated near the surface;

• Link gem chemistry to heat-change effects

Such work could begin to separate specimen-specific observations from reproducible material behaviour.

10. Conclusion

This exploratory investigation demonstrates that the durability of a lab gem cannot be described by a single property.

The lab ruby and blue cubic zirconia both resisted the common metallic objects tested and both survived a drop of approximately 5 ft onto laminate flooring without visible damage.

More aggressive abrasion revealed a clearer difference. Both materials were affected by rotary abrasives, but the cubic zirconia sustained substantially greater surface damage than the ruby. In direct contact testing, the ruby scratched the CZ, while the CZ did not visibly scratch the ruby. These observations are consistent with the greater hardness of corundum.

Both specimens nevertheless shattered under severe hammer impact, demonstrating why high hardness should not be confused with resistance to fracture.

The subsequent heating experiment revealed a different form of material behaviour. Two blue cubic zirconia specimens developed dramatically different colours under intense torch heating, despite both remaining recognisable as solid gem material.

Taken together, the observations demonstrate three useful principles:

1) hardness is not tenacity

2) physical survival is not the same as colour stability,

and 3) visual similarity does not guarantee identical material behaviour.

For jewellers, creators, and gemologists, these distinctions matter.

For Gemdrop® Research, the unexpected colour transformations also provide a future research question: what changes take place inside the coloured cubic zirconia when heat changes the colour we see?

Photographic Record

Specimens Before Testing

Workshop Tools Used in Round 1

400-Grit Polishing Disc

Faint surface scuffing was observed on both specimens.

White-Stone Burr

Both specimens showed surface damage and material loss, with substantially greater damage observed on the cubic zirconia.

Diamond Burr

Further surface abrasion occurred on both specimens, again with greater damage to the cubic zirconia.

Ruby–Cubic Zirconia Scratch Test

The ruby produced a visible scratch on the cubic zirconia. No visible reciprocal scratch was observed on the ruby.

Hammer Test

Both specimens shattered following two strikes with a 16 oz hammer.

Blue CZ Before and After Heating

Following direct torch heating, the dark blue CZ developed a deep amber-brown colour.

Pale Blue CZ After Heating

Following heating, the pale blue CZ (a unheated CZ is shown for reference on the left hand side) developed a pale seafoam-to-olive green colour (marquise cut on the right hand side).

References

Klein, C. and Dutrow, B., 2007. Manual of Mineral Science. John Wiley & Sons.

Gemdrop® Research
Scientific investigation of lab gem materials.

Experimental work and observations: India Stephens.

Scientific interpretation and research: Gemdrop®. © 2026 Gemdrop®. All rights reserved.