Hardness Is Not the Same as Indestructibility
Diamond is often described in a single sentence:
Diamond is the hardest natural mineral.
The statement is correct in terms of resistance to scratching, but it can easily lead to the mistaken conclusion that diamond cannot be broken, chipped, or damaged by heat.
It can.
The actual durability of a gem material must be divided into at least three properties:
- hardness — resistance to scratching and abrasion;
- toughness — resistance to breaking, cracking, and chipping;
- stability — behavior when exposed to heat, chemicals, and environmental changes.
Diamond is exceptionally hard and highly stable under ordinary conditions, but its toughness is not the same in every direction.
[VISUAL 2.1: The durability triangle—hardness, toughness, and stability]
Hardness: resistance to scratching
Diamond has a value of 10 on the Mohs scale of hardness. The Mohs scale is relative: it indicates which mineral can scratch another, but its numbers do not represent equal measurement intervals.
The difference between corundum at 9 and diamond at 10 is therefore not “one equal step” like centimeters on a ruler.
Instrumental methods such as Knoop and Vickers microhardness testing are available for more precise technical measurements of hardness. In gemological practice, however, the most important point is to understand what a Mohs value of 10 does—and does not—mean.
It does not indicate:
- how well a stone will withstand an impact;
- where it will cleave most readily;
- how it will react to sudden heating;
- whether a particular inclusion presents a mechanical risk;
- whether a treatment on the stone is stable.
Myth or fact? Myth: A hardness of 10 means that diamond is indestructible. Fact: Hardness primarily describes resistance to scratching. Resistance to fracture is a different property.
Hardness is not the same in every direction
Diamond exhibits hardness anisotropy: different crystallographic directions do not offer equal resistance during processing.
This is one reason diamond can be polished with diamond abrasive. A cutter must understand crystal orientation because an individual facet may polish effectively in one direction but only with great difficulty in another.
This fact connects the atomic structure discussed in the previous chapter with the practical cutting and polishing covered in Part III.
Toughness and cleavage
Toughness describes how strongly a material resists the initiation and propagation of a crack.
Diamond has perfect cleavage along octahedral planes—that is, four equivalent cleavage directions in a cubic crystal. A strong impact unfavorably oriented relative to one of these planes can cause material to separate.
This does not mean that diamond breaks easily whenever it is struck. A particular combination of factors is required:
- force;
- point of impact;
- direction;
- stone geometry;
- existing cracks or other stress concentrators.
Several phenomena must be distinguished:
- cleavage — a break that follows a favorable crystallographic plane;
- fracture — a break that need not follow an ideal cleavage plane;
- chip — localized loss of part of the material;
- abrasion — fine wear along edges or surfaces;
- bruise — impact damage that may include small radial fractures.
A detailed atlas of these characteristics appears in Chapters 45 and 47. Here, it is enough to understand that hardness and resistance to fracture are not the same property.
[VISUAL 2.2: Hardness versus fracture—scratch, cleavage, fracture, and chip]
Where a polished diamond is most vulnerable
Risk increases where force is concentrated over a small area or where a weakened zone already exists.
Particular care is required around:
- very thin sections of the girdle;
- exposed points on pear and marquise shapes;
- corners on princess and other angular shapes;
- surface-reaching fractures;
- existing chips and cavities;
- areas under strong pressure from the setting.
A clarity grade alone does not provide a complete durability assessment. A small characteristic in an unfavorable location can present greater mechanical risk than a larger inclusion situated safely within the stone.
The setting changes the risk
A good setting is not purely aesthetic. It must protect the stone mechanically while holding it securely.
Points and corners can be protected with suitable prongs or bezel settings. A worn, bent, or overly thin prong increases the risk of losing the stone. Excessive pressure during setting can also cause chipping.
Diamond’s hardness has another side: if a stone is loose, it can abrade the metal holding it. Movement of a stone within its setting is therefore not harmless.
The construction of settings, workshop procedures, and repairs are discussed in Chapters 83–87.
Stability: chemicals and heat
Under ordinary conditions, diamond is highly chemically stable and resistant to most acids it may encounter in standard gemological and jewelry practice.
Stability, however, is not absolute.
Diamond is carbon, and in the presence of oxygen it can oxidize at a sufficiently high temperature. In its practical care guidance, GIA states that diamond can begin to burn in air at about 850 °C. This figure is meaningful only when the conditions are specified: atmosphere, temperature, exposure time, and the condition of the individual stone all affect the outcome.
The following are particularly important in the workshop:
- direct heat from a torch;
- sudden temperature changes;
- existing fractures;
- different rates of thermal expansion in inclusions and the surrounding diamond;
- treatments that may be more sensitive than the diamond itself.
[VISUAL 2.3: Diamond stability—normal wear, thermal shock, and oxidation at very high temperature in air]
A treatment may be the weaker link
When a diamond has been treated, the durability of the jewelry is not determined by the diamond lattice alone.
Fracture filling and surface coatings, for example, may impose limitations on heating, cleaning, or repolishing. Before a repair or more aggressive cleaning, it is therefore necessary to know what has previously been done to the stone.
The detailed stability of individual treatments is covered in Part VIII. The general rule here is:
The stability of diamond is not automatically the stability of every treated diamond.
Diamond in everyday wear
Diamond is exceptionally well suited to jewelry that is worn frequently, but longevity also depends on the construction of the jewelry and how it is used.
Risky situations include:
- strong impacts against metal, stone, or ceramic;
- working with heavy tools;
- activities in which a ring is heavily loaded or deformed;
- contact between multiple diamond pieces when they are not kept separate;
- continued wear after a stone begins to move.
If a loose stone, bent prong, new chip, or deformed setting is noticed, the jewelry should no longer be worn until it has been inspected. Periodic professional inspection is reasonable for frequently worn jewelry; the appropriate frequency depends on the construction, wearing habits, and prior condition.
Diamond can scratch diamond
One diamond can scratch another. Diamond jewelry should therefore not be stored in a way that allows stones to make uncontrolled contact with one another or with softer gems and precious metals.
This is a simple consequence of the property that also makes diamond exceptionally useful in industry: its hardness also matters when that contact is undesirable.
A view through the loupe
When durability is assessed under magnification, particular attention is given to:
- surface-reaching fractures;
- chips and cavities along the girdle;
- edge abrasion;
- damage to points and corners;
- signs of pressure from the setting;
- the condition of prongs and other stone-retaining elements.
A loupe does not provide a complete three-dimensional map of stress. More complex cases may require a microscope, different lighting, or laboratory analysis.
Chapter summary
- Hardness, toughness, and stability are three different aspects of durability.
- Diamond is the hardest natural mineral in terms of resistance to scratching.
- The Mohs scale is relative and not linear.
- Diamond’s hardness depends on crystallographic direction.
- Diamond has perfect octahedral cleavage and can break when struck unfavorably.
- Points, corners, very thin girdles, and surface-reaching fractures require particular care.
- A clarity grade is not the same as an assessment of mechanical safety.
- A setting can protect a stone, but it can also endanger it if worn, deformed, or excessively tight.
- In air and at very high temperature, diamond can oxidize; GIA gives about 850 °C as a practical reference point.
- Treatments may have lower thermal or chemical stability than diamond itself.
- Diamond can scratch another diamond and wear away the metal of its own setting.
- The longevity of jewelry depends on the stone, setting, wearing habits, and maintenance.
[VISUAL 2.4: Mechanical risk map—round, pear, marquise, princess, and emerald shapes]