Step 1: Introduction to Gemology
Table of Contents:
- What Does Gemstone Hardness Mean?
- The Mohs Scale of Hardness
- Hardness Ranges
- What Does Gemstone Wearability Mean?
- Wearability Grades
- Gemstone Cleavage
- Jewelry Settings
Did you know that the particles of dust you see floating in the air and settling on tables contain quartz? This mineral has a hardness of 7 to 7.5 on the Mohs scale. That means the quartz in dust makes those particles hard enough to actually remove the finish from your car and the polish from your table. Dust can even cut glass. Everyday hazards such as this make gemstone hardness an important consideration when designing and wearing jewelry.
You may know that diamond, with a hardness of 10, is the hardest material in the gem world. However, there’s an old saying among gemologists:
If you hit a diamond with a hammer, it’ll shatter into a dozen pieces. Hit a piece of quartz with a hammer, it’ll split in two. Hit a piece of jade with a hammer, it’ll ring like a bell!
How is that possible if diamond is the hardest gem?
What Does Gemstone Hardness Mean?
Gemstone hardness is a very misunderstood property. The word “hardness” has a very specific scientific meaning in gemology that differs considerably from its everyday usage. The scientific definition of hardness is the ability to resist scratching, nothing more. If you ask most folks, they’ll say feathers are soft and glass is hard. In the world of gemology, however, glass is fairly soft. A variety of what we gemologists consider hard substances could easily scratch it.
Hardness depends on the bonds that hold the atoms together within a crystal structure. This bonding is evident in the ease with which the layers of atoms at a surface can be separated by applying pressure with a sample of another material. If the second material is harder than the first, it’ll leave a furrow, or scratch. That represents the breaking of millions of atomic bonds on a microscopic scale. The hardness of a mineral is, specifically, its “scratchability.” Every mineral can be ranked based on those others minerals it can scratch.
The Mohs Scale of Hardness
In 1812, the mineralogist Friedrich Mohs (1773-1839) established a reference scale of ten common minerals, ranked in order of increasing hardness. This scale, shown below, is called the Mohs scale of hardness.
|9||Corundum (rubies and sapphires)|
|7||Quartz [Example: It scratches window glass]|
|6||Feldspar [Example: A steel file will scratch it]|
|4||Fluorite [Example: A knife will scratch it]|
|3||Calcite [Example: A copper coin will scratch it]|
|1||Talc [Example: A fingernail will scratch it]|
I’ve added some examples of other familiar items to help illustrate the concept.
Each of these minerals can be scratched by the one above it and will scratch the ones below it. Minerals of the same hardness won’t scratch each other. Thus, a ruby can’t scratch a sapphire and vice versa. A diamond is much harder than corundum, even though they’re only one division apart on the scale. The Mohs scale starts approximately linear, but the curve climbs sharply at the high end. Corundum (9) is twice as hard as topaz (8), and diamond (10) is four times as hard as corundum. (Note: diamonds, and only diamonds, can scratch other diamonds).
The hardness of a material may vary slightly with composition and also with its state of aggregation. Measuring gemstone hardness can be tricky. Often, a mark that looks like a scratch is actually a trail of powder left by the supposedly harder material! Fractional hardness scores, like 5.5, are reported when the literature has indicated an intermediate value. However, it’s really not critical whether the hardness of a mineral is 5 or 5.5. A hardness range is much more meaningful.
The values reported on this website represent all values encountered in the literature. In only one case, kyanite, does the hardness of a mineral vary very widely, even within a single crystal. In most cases, the hardness range reported is very small (one unit).
Going back to the old saying, a diamond is both hard, defined scientifically, and brittle. The steel of a hammer (hardness 5 or 6) won’t scratch a diamond, but it can shatter a diamond.
“Tenacity” measures a material’s resistance to blows. For gemstones, these measurements are usually given as descriptive terms. As it so happens, most gemstones would be considered “brittle.” There are a few exceptions. Cryptocrystalline quartz, such as chalcedony, has “tough” tenacity. Jade, both jadeite and nephrite, has “very tough” tenacity.
What Does Gemstone Wearability Mean?
Assuming you don’t regularly bash your jewelry with a hammer, scratching is a hazard encountered more frequently. Think of how many times every day you put your hands inside pockets, purses, glove compartments, and desk drawers. Now think of what brushes against a ring when you do those things.
Gemstone hardness contributes greatly to the degree to which a gem will show wear. This is often referred to as wearability or sometimes “durability.” An opal with a hardness of 6 will be rapidly covered with fine scratches and lose its polish if worn everyday as a ring stone. Remember, quartz (hardness 7) is a component of dust and one of the most common minerals on Earth. Simply wiping off dust from a softer material will scratch it. The scratches may be tiny, even microscopic, but they’ll accumulate and become visible over time. On the other hand, a ruby with a hardness of 9 will remain bright and lustrous for years because it’s harder than most of the abrasive particles that contribute to wear.
In practical terms, gems softer than quartz will lose their polish and become dull simply from cleaning. A 7 or higher on the Mohs scale usually indicates a gem is hard enough for normal jewelry use. This is a guideline, not a hard and fast rule. Pearls and opals are some of the most popular jewelry gems but are well below 7.
Wearability can be graded as Excellent, Very Good, Good, Poor, or Display Only. An “Excellent” grade means a stone can be worn in virtually any type of setting for any occasion, even daily wear. A stone with a wearability grade of “Poor,” like an opal, means its jewelry use should be very carefully considered. You should only wear it occasionally or in protective settings. “Display Only” stones are for collections or display purposes only.
Although gemstone hardness has a significant effect on wearability, other factors play a role, too. Just because a gem is hard doesn’t automatically mean it will wear well. Some gems are sensitive to changes in temperature or common chemicals, even sweat. One of the most important factors is gemstone cleavage.
Technically, cleavage has to do with how strongly the molecules of a gemstone bind to each other. To put it in lay terms, it’s much like wood grain. You can easily split a piece of wood along the grain, but going across the grain is much more difficult. Many gems have cleavage planes, which vary across gem species. Cleavage planes are defined as perfect, good, fair, or poor, depending on how easily the mineral will separate along the plane. (Some gems lack cleavage planes).
Have you ever seen diamond cutters in old movies laboring over large diamond crystals? Cautiously, they chisel the crystals with brisk and carefully measured blows. If done just right, the result is two perfect pieces to cut into fabulous gems. If done poorly, the diamond shatters.
Diamond cutters don’t have to do that anymore, but this is a good example of the effect of gemstone cleavage. Diamonds are the hardest substance in nature but also have perfect cleavage. This means they can easily split along cleavage planes. This is a boon for cutting if done properly. Unfortunately, this also means diamonds can chip or shatter with wear. On the other hand, quartz is less hard than diamond but can take a lot of banging about without damage.
Jewelry settings are another important factor for gemstone wearability. Rings are perhaps the most popular jewelry use for gems. However, rings also receive more abuse than any other form of jewelry. Stones with a wearability grade of Poor (like moonstone) would be better suited for pendants, earrings, or brooches.
If you really want your soft stone in a ring, consider the following options:
- Reserve the ring for formal wear instead of daily wear. This will significantly reduce the damage that will accumulate over the years.
- Avoid an exposed ring setting. A Tiffany setting that holds the gem well above the finger with a few prongs just asks for trouble. The stone itself will come into contact with a variety of materials that may scratch it, even during occasional wear.
- Opt for a protective setting that surrounds the gem with a lot of metal. Gold is soft but easily polished and fairly easy to replace when it wears away. You likely can’t say the same for your gemstone!
Gemstone hardness alone isn’t a measure of a stone’s wearability. Factors such as tenacity, environmental sensitivity, cleavage, and settings must also be considered to determine how well a gem will wear. In our individual Gem Listings, we include wearability grades in addition to hardness measurements for many gemstones.