A Beginner's Guide to Mineral Physical Properties: Hardness, Cleavage, Crystal Systems and More
Understanding Minerals: Physical Properties
Part one of our Mineralogy Guide series. This guide covers the physical properties you can observe, test, and feel when handling a mineral specimen. No specialist equipment required for most of these — just attention and curiosity.
Mohs Scale of Hardness
Hardness in mineralogy refers to a mineral’s resistance to being scratched, and it is one of the most useful and accessible properties for identifying and understanding specimens in a collection.
The Mohs Scale is the standard measure of mineral hardness, developed in 1812 by the German mineralogist Friedrich Mohs. It ranks minerals on a scale from 1 to 10, where each mineral can scratch any mineral ranked below it and will be scratched by any mineral ranked above it. The scale is not linear: the difference in absolute hardness between consecutive numbers is not equal across the scale. Diamond at 10 is not twice as hard as Corundum at 9. It is roughly four times harder. The scale is ordinal, meaning it tells you the order of hardness but not the magnitude of the differences.
The ten reference minerals on the scale are Talc at 1, Gypsum at 2, Calcite at 3, Fluorite at 4, Apatite at 5, Orthoclase Feldspar at 6, Quartz at 7, Topaz at 8, Corundum at 9, and Diamond at 10.
A few everyday objects serve as useful reference points. A fingernail scratches at approximately 2.5, a copper coin at around 3.5, a steel knife blade at around 5.5, and a glass plate at around 5.5 to 6. This means you can estimate the hardness of an unknown mineral using objects you already have to hand before reaching for a reference mineral.
Hardness is directional in some minerals. Kyanite is the most famous example, measuring approximately 4.5 along the length of the crystal and 6.5 to 7 across it. This directional variation, known as anisotropic hardness, arises from the internal crystal structure and is itself a useful identification characteristic.
It is worth noting that hardness and toughness are not the same thing. A mineral can be very hard and still be fragile if it has perfect cleavage or a brittle tenacity. Diamond is the hardest mineral on Earth but can be cleaved or shattered by a well-placed blow. Jade, significantly softer, is one of the toughest minerals known because its interlocking microcrystalline structure resists fracture extremely well.
Cleavage
Cleavage is the tendency of a mineral to break along flat, planar surfaces that reflect the internal arrangement of atoms within the crystal structure. These planes of weakness exist because the atomic bonds are weaker in certain directions than in others, and when stress is applied the mineral preferentially splits along those planes rather than randomly.
Cleavage is described by two characteristics: the number of directions in which it occurs and the quality of each direction.
The number of cleavage directions varies from one to six depending on the mineral. Micas have perfect cleavage in one direction only, which is why they split into thin flat sheets so readily, Green Fuchsite is a beautiful example of this in action. Feldspars have two directions of cleavage at roughly right angles. Calcite has three directions of cleavage that do not meet at right angles, producing the characteristic rhombohedron fragments when broken. Fluorite has four directions of cleavage producing octahedral fragments. Sphalerite has six.
Cleavage quality is described as perfect, good, distinct, indistinct, or poor. Perfect cleavage produces flat, mirror-like surfaces that can be mistaken for crystal faces. Good cleavage produces flat surfaces but with some irregularity. Indistinct or poor cleavage produces surfaces that are barely distinguishable from a general fracture.
Cleavage has direct practical implications for collectors. A mineral with perfect cleavage in multiple directions is vulnerable to splitting if knocked or dropped, regardless of its hardness. Topaz, for example, has a hardness of 8 but perfect basal cleavage, making it considerably more fragile in practice than its hardness suggests. This is why understanding both hardness and cleavage together gives a more complete picture of how robust a specimen actually is.
Fracture
Fracture describes how a mineral breaks when it does not break along a cleavage plane. Where cleavage produces flat, planar surfaces controlled by crystal structure, fracture produces irregular surfaces whose character depends on the physical properties of the material rather than its internal geometry.
The most important fracture type for collectors is conchoidal fracture, which produces smooth, curved surfaces resembling the inside of a shell. It is the characteristic fracture of glass, flint, and obsidian, and it occurs in any amorphous or fine-grained material where there are no planes of weakness to control the break. Conchoidal fracture produces extremely sharp edges, which is why Obsidian and flint were so valuable to our ancestors as toolmaking materials.
Other fracture types include uneven or irregular fracture, which produces rough, irregular surfaces with no consistent curvature, common in many massive minerals. Splintery or fibrous fracture produces elongated splinters or fibres, seen in minerals with a fibrous habit such as some Selenite varieties and fibrous Serpentine. Hackly fracture produces jagged, sharp, irregular surfaces resembling torn metal and is characteristic of native metals such as copper and silver.
Many minerals display a combination of cleavage and fracture depending on the direction and nature of the stress applied. Understanding both properties together gives a more complete picture of how a mineral will behave when handled or worked.
Tenacity
Tenacity describes how a mineral responds to mechanical stress: specifically how it behaves when you attempt to break, bend, cut, crush, or deform it. It is a distinct property from hardness, which measures resistance to scratching, and from cleavage, which describes preferential planes of breakage.
The principal tenacity terms used in mineralogy are as follows.
Brittle minerals shatter or crumble when struck or stressed. Most silicate and oxide minerals are brittle, and this is the most common tenacity in the mineral world. A brittle mineral will produce powder or angular fragments when crushed.
Elastic minerals can be bent and will spring back to their original form when the stress is released. This is the characteristic tenacity of the mica group, where individual sheets can be flexed repeatedly without permanent deformation. It arises from the layered structure of phyllosilicate minerals where weak interlayer bonds allow flexing without breaking the stronger bonds within the sheets. Green Fuchsite is a good example of an elastic mica you can observe this in directly.
Flexible minerals can be bent and will stay in the bent position rather than returning to their original form, but they will not shatter. Chlorite and some talc varieties display this property.
Malleable minerals can be hammered into thin sheets without breaking. Native metals including gold, silver, and copper are malleable, and this property reflects the non-directional metallic bonding that allows atoms to slide past one another without the bond breaking.
Sectile minerals can be cut with a knife, producing a smooth cut surface rather than shattering. Gypsum and some sulphide minerals are sectile.
Ductile minerals can be drawn into a wire. This is again characteristic of native metals with metallic bonding.
Understanding tenacity is practically important for collectors because it determines how a mineral should be handled, stored, and if relevant worked. A brittle mineral with perfect cleavage requires fundamentally different handling from an elastic mica or a malleable native metal.
Crystal System
The crystal system of a mineral describes the fundamental symmetry of its internal atomic arrangement, which in turn controls the overall geometry of well-formed crystals and many of the mineral’s physical properties including cleavage directions, optical behaviour, and the presence or absence of pleochroism.
All crystalline minerals belong to one of seven crystal systems, each defined by the relative lengths and angles of three imaginary reference axes that describe the symmetry of the structure.
The cubic system, also called isometric, has three axes of equal length meeting at right angles. Minerals in this system include Garnet, Spinel, Fluorite, Halite, and Diamond. Cubic minerals are optically isotropic, meaning light travels through them identically in all directions, and they therefore cannot display pleochroism or birefringence.
The tetragonal system has two axes of equal length and one of different length, all meeting at right angles. Zircon and Apophyllite are tetragonal minerals. These minerals are uniaxial, meaning they have one optical axis, and can display dichroism.
The hexagonal system has three equal horizontal axes at 120 degrees to one another and one vertical axis of different length. Beryl, Apatite, and Quartz in its broader classification belong here.
The trigonal system is sometimes treated as a subdivision of hexagonal. It includes Calcite, Tourmaline, and Corundum. Like hexagonal minerals, trigonal minerals are uniaxial and can display dichroism.
The orthorhombic system has three axes of different lengths all meeting at right angles. Topaz, Andalusite, and Tanzanite are orthorhombic. These minerals are biaxial and can display trichroism.
The monoclinic system has three axes of different lengths, two of which meet at an oblique angle while the third is perpendicular to both. Malachite, Azurite, Gypsum, and most micas are monoclinic. These are biaxial minerals capable of trichroism.
The triclinic system has three axes of different lengths all meeting at oblique angles. It is the system with the lowest symmetry. Kyanite and some Feldspars are triclinic. These minerals are also biaxial.
Crystal system is one of the first things recorded when a new mineral is described, and it underpins the classification and identification of mineral species at a fundamental level.
Crystal Habit
Crystal habit describes the characteristic overall shape that a mineral typically develops when it grows, as distinct from the crystal system which describes internal symmetry. Two minerals can belong to the same crystal system but develop entirely different habits depending on which crystal faces grow fastest and most prominently during crystallisation.
Common crystal habits and what they look like in practice are as follows.
Prismatic habits produce elongated crystals with a roughly rectangular or hexagonal cross-section, like a pencil or column. Tourmaline, Beryl, and Quartz commonly develop prismatic habits.
Tabular habits produce flat, plate-like crystals where one dimension is significantly shorter than the other two. Many Feldspars and some Barite crystals are tabular.
Bladed habits produce flat, elongated crystals like the blade of a knife. Kyanite is the classic bladed mineral.
Acicular habits produce needle-like crystals, fine and highly elongated. Natrolite and Rutile inclusions in Quartz are acicular.
Botryoidal habits produce rounded, grape-like masses of mineral. Malachite and Smithsonite commonly form botryoidal masses.
Fibrous habits produce parallel or divergent fine fibres. Satin Spar Gypsum and some Serpentine varieties are fibrous.
Massive habits describe minerals that form without any obvious crystal faces or external geometry, occurring instead as irregular lumps or fine-grained aggregates. Many commercial mineral materials including most Jasper and Chalcedony are massive.
Druzy describes a surface coating of small, well-formed crystals giving a sparkling appearance. Many cavity-lining minerals including Quartz and Cobaltoan Calcite form druzy crusts.
Habit is one of the most immediately useful properties for recognising minerals in the field or in a collection, and learning the characteristic habits of common minerals makes identification considerably more straightforward.
Streak
Streak is the colour of a mineral in powdered form, obtained by scratching the mineral across a piece of unglazed porcelain known as a streak plate. The streak plate has a hardness of approximately 6.5, so it works as a streak test only for minerals softer than that.
Streak is a more reliable identification property than body colour because the colour of a mineral in bulk can be highly variable depending on trace impurities, inclusions, and surface oxidation, while the streak colour reflects the fundamental chemistry of the mineral more consistently. Hematite is a good example: it can appear silver, red, brown, or black in different specimens, but always produces a characteristic red-brown streak regardless of the body colour. This consistency makes streak a valuable diagnostic tool.
Some minerals have a streak that matches their body colour. Malachite produces a pale green streak consistent with its body colour. Others have a streak that is dramatically different. Pyrite, which appears metallic gold and is frequently mistaken for gold by beginners, produces a greenish-black streak, while real gold produces a golden yellow streak. This difference in streak is one of the quickest ways to distinguish between the two.
Minerals harder than the streak plate cannot be tested in this way as they will scratch the plate rather than leaving a streak. For these minerals, the powder colour can sometimes be obtained by other means, but streak testing becomes impractical above hardness 6.5.
Summary
The physical properties covered in this guide: hardness, cleavage, fracture, tenacity, crystal system, crystal habit, and streak, are the foundation of mineral identification and the starting point for understanding any specimen in a collection. They can be observed or tested with minimal equipment and they tell a coherent story about the internal structure of the mineral and how it will behave in the real world. Each mineral guide in the Mineral Vault references these properties, and returning to this guide whenever an unfamiliar term appears will build a working knowledge of mineralogy that makes every specimen more rewarding to study.
For the optical properties of minerals including colour, lustre, refractive index, birefringence, and pleochroism, see our Pleochroism Guide.
As always, our inbox and DMs are open if you would like guidance or simply wish to explore further.
Love, Laura
Further Reading
- Understanding Pleochroism: How Crystal Structure Creates Colour Change in Gemstones
- Blue Kyanite: One Mineral, Two Hardnesses, and a Billion Year Story
- Malachite: From Ancient Egyptian Cosmetics to the Winter Palace
- Green Fuchsite: The Green Crystal That Could Have Been Red
- Black Obsidian: The Volcanic Glass Sharper Than Surgical Steel
- Pyrite: The Mineral That Fooled the World and Still Fascinates It
- Azurite: The Mineral That Coloured Medieval Paintings
- Tanzanite: The Gemstone Discovered in 1967 That May Run Out Within Your Lifetime
