HardnessML-01
Mohs hardness chart: the ten reference minerals, and how hard everyday objects, gems and metals are
Four tables put the reference minerals, everyday objects, gems and metals on the Mohs scale, with ranges kept as ranges and a source for every row.
At a glance
This article walks through 13 topics, in this order.
- In short: how do the ten reference minerals define the Mohs scale?
- How hard are everyday objects on the Mohs scale?
- What is the Mohs scale, in simple terms?
- What is the Mohs hardness of gems?
- What is the Mohs hardness of metals?
- Is anything harder than 10 on the Mohs scale?
…and 7 more in the contents below.
In this article (13 sections)
- In short: how do the ten reference minerals define the Mohs scale?
How hard are everyday objects on the Mohs scale?3
What is the Mohs scale, in simple terms?1
What is the Mohs hardness of gems?2
What is the Mohs hardness of metals?1
- Is anything harder than 10 on the Mohs scale?
- What ranks highest and lowest on the Mohs scale?
How is Mohs hardness different from Vickers hardness?1
- What is the Mohs hardness of an Azuki Bar?
How can I test Mohs hardness myself?1
- What should I watch out for when using a Mohs hardness chart?
- Sources
The Mohs scale ranks how well things resist being scratched, from 1 to 10.
It is built on ten reference minerals. The softest, talc, is 1. The hardest, diamond, is 10.
To place something on the scale, you check which reference minerals it can scratch.
The key point: the Mohs scale tells you which is harder, not how many times harder.
In this article:
- The ten reference minerals (table)
- How hard everyday things are: fingernails, coins, glass, teeth
- 21 gems and several metals
- What Mohs 5, 6 and 7 compare to in daily life
- Whether anything is harder than 10
- How Mohs hardness differs from Vickers hardness
- How to test hardness yourself, and what to watch out for
The values were checked against the Handbook of Mineralogy (Mineralogical Society of America), GIA (Gemological Institute of America) and a university mineralogy lab handout.
In short: how do the ten reference minerals define the Mohs scale?
The Mohs scale is defined by the ten minerals below. 1 is the softest and 10 is the hardest.
| Mohs | Mineral | Measured | Everyday guide |
|---|---|---|---|
| 1 | Talc | 1 | Feels soapy to the touch |
| 2 | Gypsum | 1.5–2 | Scratched by a fingernail (2.5) |
| 3 | Calcite | 3 | Copper coin (3) |
| 4 | Fluorite | 4 | Scratched by a steel nail (5) |
| 5 | Apatite | 5 | Steel nail (5) |
| 6 | Orthoclase | 6–6.5 | Scratches glass (5.5) |
| 7 | Quartz | 7 | Slightly softer than an unglazed streak plate (7.5) |
| 8 | Topaz | 8 | No common tool scratches it |
| 9 | Corundum | 9 | The mineral of ruby and sapphire |
| 10 | Diamond | 10 | The hardest reference |
Source (10)
- Handbook of Mineralogy, "Talc" (Mineralogical Society of America)
- Handbook of Mineralogy, "Gypsum" (Mineralogical Society of America)
- Handbook of Mineralogy, "Calcite" (Mineralogical Society of America)
- Handbook of Mineralogy, "Fluorite" (Mineralogical Society of America)
- Handbook of Mineralogy, "Fluorapatite" (Mineralogical Society of America)
- Handbook of Mineralogy, "Orthoclase" (Mineralogical Society of America)
- Handbook of Mineralogy, "Quartz" (Mineralogical Society of America)
- Handbook of Mineralogy, "Topaz" (Mineralogical Society of America)
- Handbook of Mineralogy, "Corundum" (Mineralogical Society of America)
- Handbook of Mineralogy, "Diamond" (Mineralogical Society of America)
The "Mohs" column is the value the scale assigns. The "Measured" column is the value for the real mineral in the Handbook of Mineralogy.
Gypsum and orthoclase show a small gap between the two. It helps to read the scale value as the name of a mark on a ruler, while the real mineral has a range.
The last column gives an everyday object of similar hardness. The next section puts everyday objects in one table.
How hard are everyday objects on the Mohs scale?
On the Mohs scale, a fingernail is 2.5, a copper coin 3 and glass 5.5.
| Object | Mohs | Basis of the value |
|---|---|---|
| Pencil lead (graphite) | 1–2 | Value for graphite. Pencil lead is graphite mixed with clay |
| Table salt crystal | 2–2.5 | Value for the mineral halite |
| Amber | 2–2.5 | Gem value |
| Fingernail (untreated) | 2.5 | Value for the object itself |
| Pearl | 2.5–3 | Gem value |
| Copper coin (US cent before 1982) | 3 | Value for the object itself |
| Steel nail | 5 | Value for the object itself |
| Tooth enamel | 5 | Value for hydroxylapatite, its main mineral |
| Pocket knife blade | 5–5.5 | Value for the object itself |
| Glass plate | 5.5 | Value for the object itself |
| Steel file | 6.5 | Value for the object itself |
| Quartz grains in sand | 7 | Value for quartz |
| Streak plate (unglazed porcelain) | 7.5 | Value for the object itself |
Source (7)
- Handbook of Mineralogy, "Graphite" (Mineralogical Society of America)
- Handbook of Mineralogy, "Halite" (Mineralogical Society of America)
- GIA, "Amber"
- Texas A&M International University, "Lab 1: Physical Properties and the Identification of Nonsilicate Minerals"
- GIA, "Pearl"
- Handbook of Mineralogy, "Hydroxylapatite" (Mineralogical Society of America)
- Handbook of Mineralogy, "Quartz" (Mineralogical Society of America)
The rows for the fingernail, coin, nail, knife, glass, file and streak plate are the tools geology students use in mineral labs.
You do not need a box of reference minerals. A fingernail, a nail and a piece of glass already give you a rough reading.
Check the "Basis of the value" column as well. For table salt and teeth, the table gives the value of their main mineral, not a measurement of the object itself.
If your fingernail scratches it, it is 2.5 or less. If a steel nail scratches it, it is 5 or less. Start by sorting with these two.
What do Mohs 5, 6 and 7 compare to in everyday life?
Mohs 5 is about a steel nail, 6 is a little harder than glass, and 7 is a grain of quartz sand.
- 5: steel nail; the main mineral of tooth enamel
- 5 to 5.5: pocket knife blade
- 5.5: glass plate
- 6: orthoclase (scratches glass)
- 6.5: steel file
- 7: quartz (rock crystal), grains of sand
Stones softer than 7 can be scratched when sand grains rub against them. That is why 7 is often used as a rule of thumb when handling gems.
How hard is a 10-yen coin on the Mohs scale?
No public source gives a measured Mohs hardness for the 10-yen coin (十円玉) itself.
The 10-yen coin is bronze made of 95% copper, according to the Bank of Japan.
The closest guide is the "copper coin" in the university handout, at 3. That figure is for an old US one-cent coin, though.
The 10-yen coin has a different alloy and is made differently. Treat 3 as a rough guide: about as hard as calcite.
How hard are fingernails and teeth on the Mohs scale?
A human fingernail is 2.5 on the Mohs scale, measured on an untreated nail.
Nail polish or nail hardener can make a nail harder than that. Use a bare nail when you test hardness.
Tooth enamel is mostly made of a mineral called hydroxylapatite, which has a hardness of 5.
Hydroxylapatite belongs to the same group as apatite, the reference mineral for 5. It is reasonable to think of tooth enamel as about 5 on the Mohs scale.
What is the Mohs scale, in simple terms?
The Mohs scale is an order of hardness: rub two things together, and the one that gets scratched is the softer.
The mineralogist Friedrich Mohs chose the ten minerals. GIA dates the scale to 1822. Some sources give 1812.
This is how a Mohs value is found:
- Rub the thing you are testing against a reference mineral
- If the reference mineral gets scratched, your sample is harder
- If it does not, your sample is softer or equally hard
- Place your sample between the reference minerals above and below it
For example, something that scratches fluorite (4) but not apatite (5) is "between 4 and 5".
Two things of the same hardness scratch each other. The Mohs scale is a way to compare, and it needs no measuring machine.
Are the steps on the Mohs scale equal?
No. The steps are not equal, and a difference of one does not mean the same difference in hardness everywhere on the scale.
GIA explains that diamond at 10 is many times harder than corundum at 9.
The university handout also says that a mineral of hardness 2 is not twice as hard as one of hardness 1. The numbers only show the order.
Do not read "7 is 2 harder than 5". It is a ranking, so you cannot subtract or divide the numbers.
What is the Mohs hardness of gems?
Among gems, diamond is the hardest at 10, while pearl and amber are at the low end at about 2 to 3.
| Gem | Mohs | Sp. gravity | Mineral |
|---|---|---|---|
| Diamond | 10 | 3.52 | Diamond |
| Ruby, sapphire | 9 | 4.00 | Corundum |
| Alexandrite | 8.5 | 3.73 | Chrysoberyl |
| Spinel | 8 | 3.60 | Spinel |
| Topaz | 8 | 3.49 | Topaz |
| Emerald | 7.5–8 | 2.72 | Beryl |
| Aquamarine | 7.5–8 | 2.72 | Beryl |
| Zircon | 7.5 | 4.6 | Zircon |
| Tourmaline | 7–7.5 | 3.06 | Tourmaline |
| Amethyst | 7 | 2.66 | Quartz |
| Garnet | 6.5–7.5 | 3.47–4.15 | Garnet group |
| Jadeite jade | 6.5–7 | 3.34 | Jadeite |
| Peridot | 6.5–7 | 3.34 | Olivine |
| Tanzanite | 6–7 | 3.35 | Zoisite |
| Nephrite jade | 6–6.5 | 2.95 | Tremolite, actinolite |
| Moonstone | 6–6.5 | 2.58 | Feldspar |
| Opal | 5–6.5 | 2.15 | Opal |
| Turquoise | 5–6 | 2.76 | Turquoise |
| Lapis lazuli | 5–5.5 | 2.38 | Value for lazurite, its main mineral |
| Pearl | 2.5–3 | 2.60–2.85 | Not a mineral (made by molluscs) |
| Amber | 2–2.5 | 1.08 | Not a mineral (fossil resin) |
Source (20)
- GIA, "Diamond"
- GIA, "Ruby"
- GIA, "Alexandrite"
- GIA, "Spinel"
- Handbook of Mineralogy, "Topaz" (Mineralogical Society of America)
- GIA, "Emerald"
- GIA, "Aquamarine"
- Handbook of Mineralogy, "Zircon" (Mineralogical Society of America)
- GIA, "Tourmaline"
- GIA, "Amethyst"
- GIA, "Garnet"
- GIA, "Jade"
- GIA, "Peridot"
- GIA, "Tanzanite"
- GIA, "Moonstone"
- GIA, "Opal"
- GIA, "Turquoise"
- Handbook of Mineralogy, "Lazurite" (Mineralogical Society of America)
- GIA, "Pearl"
- GIA, "Amber"
The gem values come from GIA's gem pages. Where a range is given, the table keeps the range.
Gems made of the same mineral are about equally hard. Emerald and aquamarine are both beryl. Ruby and sapphire are both corundum.
Which gems are the hardest and the softest?
The hardest gems are diamond, ruby and sapphire, and alexandrite.
The softest are pearl, amber, turquoise and opal. Pearl and amber are not minerals: pearls are made by molluscs, and amber is fossil resin.
Gems below 7 scratch more easily if they are stored with dust or other gems. Keeping each piece separate is safer.
Does a harder gem break less easily?
Not necessarily. Resistance to breaking is a separate property called toughness.
In GIA's figures, the toughness of nephrite jade is 225,000. Diamond is about 5,000 to 8,000.
Diamond resists scratches, but it can break if it takes a hard knock. Nephrite scratches more easily, but it is hard to break.
Hardness is about scratches, toughness is about breaking. When you choose a ring, look at the two separately.
What is the Mohs hardness of metals?
On the Mohs scale, gold, silver and copper are 2.5 to 3, iron is 4 and platinum is 4 to 4.5.
| Metal | Mohs | Sp. gravity | Measured on |
|---|---|---|---|
| Gold | 2.5–3 | 19.3 | Native metal |
| Silver | 2.5–3 | 10.1–11.1 | Native metal |
| Copper | 2.5–3 | 8.95 | Native metal |
| Iron | 4 | 7.3–7.87 | Native metal |
| Platinum | 4–4.5 | 14–19 | Native platinum (impure) |
| Steel nail | 5 | Steel tool | |
| Pocket knife blade | 5–5.5 | Steel tool | |
| Steel file | 6.5 | Steel tool | |
| Pyrite (looks like gold) | 6–6.5 | 5.02 | Iron sulfide mineral |
Source (7)
- Handbook of Mineralogy, "Gold" (Mineralogical Society of America)
- Handbook of Mineralogy, "Silver" (Mineralogical Society of America)
- Handbook of Mineralogy, "Copper" (Mineralogical Society of America)
- Handbook of Mineralogy, "Iron" (Mineralogical Society of America)
- Handbook of Mineralogy, "Platinum" (Mineralogical Society of America)
- Texas A&M International University, "Lab 1: Physical Properties and the Identification of Nonsilicate Minerals"
- Handbook of Mineralogy, "Pyrite" (Mineralogical Society of America)
The values for gold, silver, copper, iron and platinum are for native metals found in nature, which are nearly pure metal minerals.
Everyday metal tools are made of steel, which is iron with carbon and other elements added. On the Mohs scale, a steel nail is 5 and a steel file is 6.5.
Pyrite is a mineral that looks a lot like gold. It is 6 to 6.5, much harder than gold at 2.5 to 3. The difference in hardness is one clue for telling gold and pyrite apart.
What is the Mohs hardness of iron, stainless steel and titanium?
Native iron is 4 on the Mohs scale. We could not find a published Mohs value for stainless steel or titanium.
The hardness of stainless steel and titanium alloys changes a lot with composition and heat treatment. That is why the hardness of metals is usually given as Vickers hardness.
To compare stainless steel or titanium, use the Vickers hardness in the manufacturer's data.
For how well these metals carry heat, see the thermal conductivity chart.
For how heavy they are, see the density of metals chart.
Gold alloys such as 18-karat gold are the same. They are harder than pure gold (2.5 to 3), but the value depends on the mix, so they are not in the table.
Is anything harder than 10 on the Mohs scale?
The Mohs scale stops at 10, and there is no reference mineral above it.
Only a diamond can scratch a diamond, as GIA also notes.
You may see values such as "Mohs 15". That comes from a different scale on which diamond was moved to 15.
Researchers working on abrasives made it in 1933, to split the gap between 9 and 10 into finer steps. It is called the modified Mohs scale.
Do not mix modified Mohs values with an ordinary Mohs chart. The same number means a different hardness.
What ranks highest and lowest on the Mohs scale?
The highest on the Mohs scale is diamond at 10, and the lowest is talc at 1.
Sorted from the tables in this article, the top of the list is:
- Diamond: 10
- Ruby and sapphire (corundum): 9
- Alexandrite: 8.5
- Spinel and topaz: 8
- Emerald and aquamarine: 7.5 to 8
At the bottom:
- Talc: 1
- Graphite in pencil lead: 1 to 2
- Gypsum: 1.5 to 2
- Table salt and amber: 2 to 2.5
How is Mohs hardness different from Vickers hardness?
Mohs hardness is an order found by scratching. Vickers hardness is a number calculated from the size of a dent.
In a Vickers test, a diamond pyramid is pressed into the surface with a set force. The hardness is calculated from the size of the dent it leaves.
The method is set out in the international standard ISO 6507 and in JIS Z 2244 in Japan.
| Mohs hardness | Vickers hardness | |
|---|---|---|
| How it is found | Whether a scratch forms | Measuring the size of a dent |
| What the number means | Order only | The size of the number matters |
| Tools | Reference minerals; fingernail, nail, glass | A testing machine |
| Main use | Minerals and gems | Metals and ceramics |
| Range | 1 to 10 | No fixed upper limit |
Can Mohs hardness be converted to Vickers hardness?
There is no fixed formula for converting Mohs hardness to Vickers hardness.
Because the Mohs scale is a ranking, one step up means a different increase at different points on the scale. So it cannot be converted with a formula.
First check whether both things you want to compare have measured Vickers values. Use Mohs hardness for a rough idea.
What is the Mohs hardness of an Azuki Bar?
There is no official Mohs value for the Azuki Bar (あずきバー) measured by a set method.
The Mohs scale is designed for minerals. Frozen food also changes hardness with temperature, so a single value is hard to pin down.
The maker, Imuraya (井村屋), has explained in an interview why the bar is so hard. It has gradually cut the sugar over the years, so there is more water to freeze.
If you see a "Mohs hardness" for the Azuki Bar with no measuring conditions, I think it is best enjoyed as a bit of fun.
How can I test Mohs hardness myself?
With a fingernail, a steel nail and a glass plate, you can find the rough Mohs range yourself.
The university handout sorts samples into three groups:
| Result | Mohs hardness | Group |
|---|---|---|
| Scratched by a fingernail | 2.5 or less | Soft |
| Not by a fingernail, but by a nail | Above 2.5, up to 5 | Medium |
| Scratches a steel nail | Above 5 | Hard |
Tips for testing:
- After rubbing, wipe off the powder and check whether it is a scratch or just powder
- Two things of the same hardness both get scratched
- The harder the sample, the more force a scratch takes
- Do not use rings or other jewelry to test
Testing with jewelry can damage the metal setting or chip the stone. The university handout also warns against using your own jewelry.
Can the same stone have a different hardness in different directions?
Yes. Some minerals change hardness with the direction you scratch them. The best-known example is kyanite.
Kyanite is 5.5 when scratched along the length of the crystal and 7 across it. Gypsum is also described as varying with direction.
Differences like this, and differences in composition, are why many values in a Mohs chart are ranges.
What should I watch out for when using a Mohs hardness chart?
Values in Mohs hardness charts can differ by 0.5 to 1 between sources, so check where each value comes from.
For example, some sources give glass as 5 and others as 5.5. There are many kinds of glass, and the sources measured different ones.
In this article, each table uses one set of sources, with a source for every row.
When you use a chart, check:
- Whether the value is for the object itself or for its main mineral
- Whether it is a single value or a range
- For alloys and processed materials, whether composition or treatment changes it
Mohs hardness: frequently asked questions
What is the highest Mohs hardness?
The highest Mohs hardness is 10, for diamond.
Values above 10 belong to other scales, such as the modified Mohs scale.
What is the lowest Mohs hardness?
The lowest is talc, the reference mineral for 1.
It feels soapy and is easily scratched with a fingernail.
How hard is Mohs 9?
Mohs 9 is the hardness of corundum, the mineral of ruby and sapphire.
Apart from diamond, it is the hardest group among natural gems. It is also used as an abrasive.
What stone is Mohs 6?
The reference mineral for 6 is orthoclase, and moonstone is 6 to 6.5.
That is hard enough to scratch glass (5.5).
Does a higher Mohs hardness mean a stone breaks less easily?
No. Hardness and resistance to breaking are separate properties.
Diamond resists scratches but can break if it takes a hard knock. Resistance to breaking is compared by toughness.
What is the Mohs hardness of stainless steel?
We could not find a published Mohs value for stainless steel.
The hardness of metals is usually compared with Vickers hardness. See the manufacturer's data.
Sources
- Handbook of Mineralogy (Mineralogical Society of America): mineral data
- Handbook of Mineralogy, "Kyanite" (hardness by direction)
- More than the Mohs Scale: Understanding Gem Durability (GIA)
- GIA Gem Encyclopedia (Gemological Institute of America)
- Lab 1: Physical Properties and the Identification of Nonsilicate Minerals (Texas A&M International University)
- Banknotes and coins currently issued (Bank of Japan, in Japanese)
- ISO 6507-1 Metallic materials: Vickers hardness test (ISO)
- Japanese Industrial Standards Committee (search for JIS Z 2244)
- Fifty years of Imuraya's Azuki Bar (Business Insider Japan, 30 June 2023, in Japanese)
- Research summary on tooth remineralization (Japan Science and Technology Agency, in Japanese)
- R. R. Ridgway, A. H. Ballard, B. L. Bailey (1933) Hardness values for electrochemical products. Transactions of the Electrochemical Society 63 (modified Mohs scale)