Gemstone Durability: Hardness, Toughness & Fracturability
Durability and Tenacity in Gemstones
The durability, or tenacity of a gem is very important to its practicality as a wearable piece of jewelry, and is more complex than any single physical feature that distinguishes the grouping of chemicals that makes up a particular gemstone.
Within the overarching terms "durability" and/or "tenacity" there are several physical aspects that are very distinct. These attributes include: hardness, cleavage, parting, brittleness/toughness, inclusions, thermal shock, gem cut, and the physical environment (gem setting and location to be worn) of the gemstone. Some gemstones may have a high hardness yet low toughness, while others have lower hardness yet higher toughness. It is because of this paradox that each attribute should be factored in separately.
Gem Hardness
Hardness in most traditional gemstones that have been faceted is not necessarily an important factor in their durability. The moh's scale had some limited use in identifying gemstones in the past, but with the advent of modern gemological equipment it is seldom used today. The testing for the moh's scale requires an attempt to scratch/damage the gemstone; something which is hardly desirable. The construction of the moh's scale is also deceiving in its limited use in gemology.
Diamond (Hardness: 10, Toughness: Good)
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Sapphire Inclusion (Hardness: 9, Toughness: Excellent)
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Having a stone that has a hardness 6 1/2 does not mean that the gem is actually equal in harnesses from 6 and 7, or just slightly less hard than 7. It just means that the gem will scratch a stone that is 6, but not scratch a stone that is 7 on the moh's scale. Where stones with a hardness between 6 and 7 fit into the relative harness scale is not given by the moh's scale.
There is really only one position on the moh's scale that is important to the durability of a gemstone in a normal environment, and that is 7, which is the hardness of quartz. Quartz is the hardest substance found in our lives outside of other gems, and in the form of micro particles it is ubiquitous. The only way to make gemstones that are softer than quartz, as durable as gemstones that are harder than quartz, is to never touch their surfaces.
Proper cleaning that minimizes contact with the stone can prolong the polish, but in time the touched areas of the stone will become dull. Continued contact with the stone, such as rubbing it during cleaning will lead to the erosion of the sharp faceted edges of the gem.
Gem Cleavage
The term "cleavage" refers to a weakness that is an intrinsic physical property (crystalline structure) of the chemical(s) that makes up the gemstone. Gems can have more than one plane of weakness (cleavage plane), any one of which can lead to the instantaneous splitting (fracturing) of the gem when enough energy is applied in the right location. The orientation and location of a gem's cleavage plane can also lead to the increased possibility of chipping along some the edges of a faceted gemstone - regardless of its hardness.
Soft & fragile Opal is subject to thermal shock
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Emerald with inclusions which effect toughness
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The purple-blue variety of zoisite known as Tanzanite has a distinct cleavage plane and is softer than quartz. This means that Tanzanite is a stone which is more prone to chipping than most traditional gemstones when worn in exposed locations such as a ring. Gemstones that are susceptible to fracturing along their cleavage planes should never be cleaned in anultrasonic cleaner.
Corundum (Sapphire and Ruby) is one of the most "durable" gemstones, with a high hardness of 9, toughness rated excellent, and no cleavage.
Parting in Gems
The term "parting" refers to a phenomenon that is similar to cleavage in that it is a "plan of weakness" that can easily cause the stone to fracture with a small amount of force. Parting is caused by the twinning of the crystal from which the gem was cut, or physical force applied to the crystal while it was being formed. Some gems are more prone to this type of weakness, but it is not universal to every sample of the chemical that makes up the gem, as is the case with cleavage.
Brittleness/Toughness in Gems
The "brittleness" or "toughness" of a stone is a measure of how much impact energy can be applied to weak points such as sharp edges of facets and corners of a gemstone without breaking. All faceted gemstones are brittle to some degree, but some varieties of gems will chip excessively; even if given proper care.
When purchasing these gems you should be informed about this potential weakness, and realize that a brittle gem may not be as durable as you would expect based on its "hardness" rating alone. This is true even if the gem is in protective jewelery settings, or will be worn in areas of the body that receive less abuse - such as a necklace. Although emerald is a "hard" gemstone with a hardness of 7.5 to 8, its toughness is rated as fair to poor.
Thermal Shock
Rapid changes in temperature can cause "thermal shock" in certain gemstones due to moisture content that is trapped within inclusions or fissures, as well as other structural and/or chemical features/defects that may be inherent in the gemstone. Any moisture that may be trapped within an inclusion can expand and contract due to extreme temperature shifts, causing fracturing. Two gemstones that are particularly effected by this type of thermal shock are opal and emerald. Opal can also be affected by low humidity which can cause fracturing. Gemstones that are susceptible to fracturing due to thermal shock should never be cleaned using a steam cleaner.
Gem Inclusions
The term "inclusions" refers to tiny imperfections such as feathers, growth tubes, included crystals or other internal flaws (all of which can be given the general title of "naturals"), will all weaken a gemstone.
Feathers & Garnet in Diamond (Photo: AGSL)
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Growth Tubes in Diamond (Photo: AGSL)
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If they are prevalent enough they can be not only be unsightly, but render the gemstone too fragile for normal use. Emerald is a gemstone variety that can be heavily included (called "Jardin" or "garden"), severely effecting its durability.
Gem Cuts & Durability
The type of cut that is used on a gemstone, and the cut's orientation within the rough stone, can render some "naturals" less distracting to the eye, and make a gem less prone to breakage. All gemstones should be checked to see that their physical structure is sound before purchase.
Exposed Ruby Facets
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Tension Setting w/ Diamond, Bezel Setting w/ Aqua
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Gem Settings
Utilizing a protective gemstone setting is a good way to ensure that you will get years of enjoyment out of a beautiful gemstone. Employing proper faceting angles and a fine polish will keep gemstones bright and scintillating, even if its setting does not allow any light into the stone from the sides or back.
The gem's use in an earring, ring, necklace, etc. is also a strong factor in the durability of a gemstone. Still, style can dictate the desired design of the setting and the gem's use, rather than the durability of the gem.
The Informed Consumer
You (or your customer) should be informed about the risks you are taking when wearing a particular gemstone, to help prolong its life and protect your investment in its beauty.
