As an Amazon Associate, I earn from qualifying purchases with no additional costs for you.
Mineral properties are very specific characteristics of every separate mineral. Being fluent in mineral properties gives the opportunity to identify any kind of mineral and even make an assumption regarding the possible use of mineral as ore, to guess what type of rock this mineral comes from, the formation environment, and even to reveal geological history of the region!
There are 23 most important mineral properties. The fundamental mineral properties are color, hardness, luster, cleavage, transparency, the morphology of chrystals, magnetism, and luminescence. There are some exotic properties like asterism, special taste or odor, and glowing under UV light.
Minerals can truly surprise you with garlic odor, can glow when hit by a hammer, or can shutter into perfect cubes! All these can be explained by mineral properties like chemical composition, luminescence, and cleavage, respectively.
And we are going to reveal them all today! Mineral properties can be grouped into 3 categories: physical properties, electrical properties, and the morphology of chrystals. We will discuss them one by one.
If you are interested in checking out the best books about rock and minerals identification you can find them by clicking here (Amazon link).
Physical Properties of Rocks & Minerals
The physical properties of rocks and minerals are basic properties that interest a person and which he starts to identify first. So let’s dive into describing physical properties:
Color
Color is the first obvious property that the human eye catches. Minerals can occur in any color, hue, saturation, and tone. To identify the color of the mineral, try to simplify it first to the most obvious one and then add a shade. For example, green -> yellowish green for beryl.
Even if the color of a mineral is a very obvious characteristic, it can be tricky, and you should be extremely careful identifying minerals by color only.
Some minerals like malachite occur in one color only – green, but quartz or tourmaline occur in a rainbow of color. That is why the nature of mineral colors can be divided into 3 groups:
- Idiochromatic minerals are “self-colored” due to their composition. The color is a constant and predictable property of the mineral. Examples are blue azurite, red cinnabar, and green malachite.
- Allochromatic minerals obtain their color due to the impurities in their composition or defects in their structure. In this case, the color is an unpredictable property of the mineral. Examples are blue in sapphire (pure corundum is colorless), produced by iron and titanium impurities, and yellow in heliodor (yellow beryl).
- Pseudochromatic minerals are “false-colored” due to light diffraction or scattering. In these cases, color is variable but a unique property of the mineral. Examples are the colors produced by precious opal and the sheen of moonstone.
It is necessary to discuss a list of optical effects which influence the mineral color. Some of them can be observed in different types of minerals, but others are typical for exclusively one type of mineral and can be used as the main identification feature.
TIP: Rock color is the first property we pay attention to. Rocks occur in a branch of colors, hues, and saturation. Find out more about the color of rocks and minerals in the article below:
Rock Colors: What Determines Color & Why Different Colors
Optical Phenomena of Minerals
Optical phenomena of minerals are among the most interesting mineral properties. Watching various optical phenomena is fun for everyone, from the youngest to the oldest rock lovers.
Pleochroism
Pleochroism is the ability to selectively absorb light waves. depending on the crystallographic direction. Minerals like andalusite, tourmaline, cordierite, kunzite, and tanzanite have strong visible pleochroism. It means the color of the mineral changes depending on the viewing angle.
Polychrome coloring
Polychrome coloring is an optical property in which the mineral has different colors within the same crystal. This property is typical for fluorite (green-white-violet), tourmaline (green-red, yellow-orange, blue-green), and quartz (a violet-yellow variety called ametrine).
Asterism
Asterism is an optical phenomenon when a four or six-ray star is observed on the mineral surface. It is created by the reflection of light from inclusions in the mineral, oriented along with certain crystallographic directions. The best examples of this property are star sapphire and star ruby.
Birefringence
Birefringence (double refraction) is the optical effect of splitting a beam of light into two components. This property can be perfectly illustrated on an Iceland spar (transparent variety of calcite). Printed text will seem doubled when observed through the crystal of the Iceland spar.
TIP: Some minerals have so strong birefringence that the picture observed through the crystal will be doubled. Find out more about double refraction in the article below:
8 Minerals That Show Double Refraction (+ Explanation Why)
Opalescence
Opalescence or flashing rainbow colors in opal is an optical phenomenon produced by the interaction of visible light with the gem’s microscopic internal structure of stacked silica spheres. This property is observed as a wonderful play-of-color, spread on the whole body of precious opal.
Labradorescence
Labradorescence is an optical phenomenon where flashes of color appear across a dark background of the labradorite mineral.
It is caused by alternating layers of two feldspars with different chemical compositions. The colors you see depend on the thickness of the alternating layers and their refractive indexes.
Color Change Effect
The color change is a very specific property confined to alexandrite (chrysoberyl) minerals. It is a very distinct change in gem color under different types of lighting.
Alexandrite looks green under daylight and red under incandescent light. Sometimes garnets, corundum, and spinel have a color change effect.
The color can be a false friend for mineral identification, and you should always be careful with the color of the mineral sample tested. Never rely on the color property as the first and the one main property to identify the mineral.
Remember, minerals with permanent color are graphite, sulfides (galena, pyrite, molybdenite), magnetite, peridot (olivine), azurite, malachite, turquoise, orpiment, and cinnabar. For these minerals, color can be a diagnostic feature.
However, halite, corundum, quartz, fluorite, garnet, beryl, topaz, tourmaline, and even diamond occur in various hues, so the color is not a reliable property for mineral identification in that case.
Streak
The streak is the color of the mineral in a powdered state. The color of the streak may differ from the color in the sample tested.
That is why the streak test is very useful for the differentiation of sulfide minerals. To get the streak, the mineral sample should be scratched on a piece of unglazed porcelain.
In most cases, the streak color matches the color of the mineral (graphite, malachite, sulfur, cinnabar). But sometimes, the color of the streak can drastically differ from the color of the mineral sample. For example, the pyrite color of crystals is brassy yellow, and the color of the streak is almost black.
However, this property also has some limits. The mineral tested should be softer than the streak plate because porcelain hardness corresponds to 5-6 on the Mohs scale. That is why for hard minerals, it is indicated that streak is absent.
TIP: Minerals’ streak is one of the most important tests for mineral identification, especially while working with native metals and sulfide minerals. Find out more in the article below:
DIY Guide: Testing Mineral’s Streak (Explained by Expert)
Transparency
Transparency is the degree to which light is transmitted through a mineral. There are three categories of transparency: opaque, translucent, and transparent.
Transparency can vary from sample to sample of the same minerals, but if, in most cases, the mineral is transparent, you will hardly find an opaque sample.
Let’s dive into transparency classification in detail:
- The opaque mineral doesn’t transmit light even in the form of very thin plates (even in thin sections). This category is represented by metal oxides and sulfides mostly (magnetite, chromite, pyrite, galenite).
- Translucent minerals transmit light only in a thin plate up to a centimeter thick (flint).
- Transparent minerals transmit light like ordinary glass (rock crystal, Iceland spar).
Luster
Luster is the ability of a mineral to reflect light. The fundamental two groups of luster are metallic (pyrite, galena, molybdenite, pyrrhotite, and hematite) and non-metallic (topaz, malachite, spodumene, gypsum, talc, and many others), which allows separating minerals for further identification.
There is no strict scientific definition of the concept of brilliance. Non-metallic luster can be further subdivided into diamond, glass, matte, oily, waxy, mother-of-pearl, iridescent, and silky luster.
Cleavage
Cleavage is the ability of crystalline minerals to split along planes of weakness in the mineral’s structure. The cleavage of many minerals is expressed in several mutually intersecting directions. It is usually specified by its quality (perfect, good, fair, poor) and its direction.
To detect cleavage, the mineral is rotated so that the surface of the cleavage reflects light into the eyes. The cleavage of many minerals is expressed in several mutually intersecting directions.
In halite and sylvite – in three directions perpendicular to each other (cleavage according to the cube). Sphalerite has six cleavage directions.
Perfect cleavage is cleavage in which the mineral splits into thin plates with a smooth shiny surface very easily (with a fingernail). Minerals with perfect cleavage are mica and talc.
Feldspar has good cleavage. Olivine has fair cleavage, which is hardly observed. Apatite and beryl have poor cleavage. It is not distinguishable.
Cleavage planes should not be confused with crystal faces. Should keep in mind that on cleavage planes, the shine is more vital than on crystal faces and other fracture surfaces.
Fracture
Fracture is a type of surface that occur after the splitting of a mineral. After shuttering, surfaces of different configurations appear. A fracture can be granular, earthy, conchoidal, splintery, stepped, and uneven. Quartz has a very distinct conchoidal (concentrically wavy) fracture.
Granular fracture is characterized by intergrown grains. Spheres are typical for oolitic limonite, and bauxite. Earthy fracture looks rough and matte. Kaolinite is a perfect example of an earthy fracture.
Conchoidal (concave, concentrically wavy) fracture can be illustrated by quartz and all its varieties. Splintery fracture is a common property of native elements (gold, silver cooper).
A stepped fracture can be observed as ledges between cleavage planes typical for halite. Uneven fracture, or randomly broken shiny surfaces, tends to solid minerals lacking cleavage (nepheline).
Hardness
The hardness of a mineral is one of the main mineral properties. It depends on its internal structure and chemical composition. The simplest way to determine hardness is to scratch one mineral with another. For such an assessment, the Mohs scale was adopted, represented by ten mineral standards.
The higher number of the mineral in the Mohs scale, the harder it is: talc – 1; gypsum – 2; calcite – 3; fluorite – 4; apatite – 5; orthoclase – 6; quartz – 7; topaz – 8; corundum – 9; diamond – 10.
It means gypsum can be scratched by any mineral, however, can scratch only gypsum. Diamond is the hardest mineral and can scratch any mineral. At the same time, only diamonds can scratch diamonds.
The hardness of the mineral sample is tested until it equals the hardness of one of the standards, or until it falls into the interval between the hardness of two neighboring standards.
For diagnostics, improvised objects are also used. The hardness of a soft pencil is 1, fingernail – 2-2,5, glass 5–6, steel needle, and a knife 6–7.
TIP: The advantage of the hardness test is that it can be easily done at home or even in the field while rockhounding. Find out more about testing the hardness of minerals in the article below:
DIY Guide: Testing Mineral’s Hardness (Explained by Expert)
Specific Gravity
Specific gravity is the ratio between the mass (weight) of a mineral and the mass (weight) of an equal volume of water. It is standardly expressed in g/cm3.
Specific gravity depends on the chemical composition and structure of the mineral. In the field, it is determined only by weighing on the hand.
The Specific gravity of minerals can be divided into three groups:
- light – less than 2.5 g/cm3 (gypsum);
- medium – from 2.5 to 5 g/cm3 (apatite);
- heavy – more than 5 g/cm3 (galena).
Most common silicate minerals have a specific gravity between about 2.5 and 3.0 g/cm3. For instance, quartz with a density of 2.65 is 2.65 times as heavy as the same volume of water.
Malleability and Brittleness
Malleable are minerals that change their shape when struck with a hammer but do not shutter. These are metals mostly (copper, gold, and silver).
Minerals that crumble into small pieces are brittle and fragile. Even though the diamond is the hardest mineral, it is very brittle and can be destroyed by a hammer.
BTW: Do you want to know more about rock and mineral identification? The books listed below are the best ones you can find on the internet (Amazon links):
- Smithsonian Handbooks: Rocks & Minerals
- Gemstone & Crystal Properties (Quick Study Home)
- Ultimate Explorer Field Guide: Rocks and Minerals (National Geographic Kids)
Electrical Properties of Rocks & Minerals
The electrical properties of rocks and minerals fascinate a large number of people, but few people understand how they are formed and what they actually mean.
Magnetism
Magnetism property is crucial for the identification of iron-bearing minerals. Magnetism is the ability of a mineral to react to small iron objects and magnets. Magnetite (iron oxide with the highest iron concentration) can attract metal pieces. Pyrrhotite can be attracted by a magnet.
Electrical Conductivity
The electrical conductivity of minerals is the ability of minerals to conduct an electric current under the influence of an electric field.
Otherwise, minerals are referred to as dielectrics, i.e. non-conductive. There are some special testers to check the electrical conductivity.
Glowing (Luminescence)
Mineral glow is a specific property when minerals glow in a response to heating, irradiation with X-rays, ultraviolet light, or even mechanical stress.
Glowing is a very helpful property for the identification of fluorite, calcite, spodumene, emeralds, rubies, and diamonds.
There are the following types of luminescence glowing) of minerals:
- Luminescence – the ability to glow at the moment of irradiation with certain rays (short or long-wave UV light). Scheelite glows blue under UV light. Calcite can glow green, red, pink, and orange.
- Phosphorescence – the ability of a mineral to glow for some time after exposure to certain rays. Willemite glows after exposure to short ultraviolet rays.
- Thermoluminescence – mineral glows when heated. Fluorite glows purple-pink.
- Triboluminescence – the phenomenon when minerals could emit light when they are mechanically stimulated by rubbing, grinding, impact, stretching, and compression. Sometimes can be observed in quartz and corundum.
TIP: Minerals and rocks, which glow under ultraviolet light, are usually perceived as a miracle. Check out the common UV glowing rocks and minerals in the article below:
12 Rocks & Minerals That Glow Under UV Light & Black Light
Other Important Mineral Properties
In this article, we have described the basic mineral properties so far. However, there are several other, interesting mineral properties that will be described in the section below.
Reaction with Hydrochloric Acid
This is a very important property when speaking about carbonate minerals. It allows you to identify the calcite at a glance. Calcite (CaCO3) and hydrochloric (HCl) acid produce visible chemical reactions, which we observe as bubbles on the surface of the mineral studied.
Flammability and Odor
Some minerals catch fire from a match and create characteristic odors (sulfur – sulfur dioxide, ozocerite – a suffocating smell of carbon monoxide).
The smell of garlic appears during griding of arsenopyrite. If pieces of phosphorite are rubbed against each other, the smell of burnt bone appears.
Radioactivity
Some minerals contain radioactive chemical elements such as uranium, thorium, tantalum, zirconium, and thorium.
They often have significant radioactivity, which is easy to detect with household radiometers. Radioactive minerals are abernathyite, gadolinite, monazite, zircon, etc.
TIP: There is a good chance that you have collected a specimen with radioactivity while out rockhounding without even knowing it. Check out common radioactive rocks and minerals in the article below:
What are Radioactive Minerals? 6 Common Radioactive Rocks
The Morphology of Crystals
Crystal morphology is the last area that needs to be completed in order to have a complete picture of the properties of rocks and minerals.
Crystal Form and Crystal Habit
Chrystal form is also sometimes included in mineral properties lists. This is a matter of debate, as sometimes mineral occurs as a massive object without any visible crystal faces. We would like to cover the crystal form topic to upgrade the identification skills of our readers.
Chrystal form is a quality of the development of crystal faces present. Crystal can be well-developed, partially developed, and irregularly formed. Well-developed crystals show a distinct habit of a mineral (the three-dimensional shape of an individual crystal), which is a useful identification feature.
Chrystal forms are divided into three groups:
- Euhedral – well-developed crystals where most crystal faces are shown. Quartz crystal, aquamarine crystal, and fluorite octahedron crystal are good examples of euhedral crystal forms.
- Subhedral crystals are partially-developed crystals with only some crystal faces shown. These can be the same minerals as in the previous example but the party altered during the mineral forming process.
- Anhedral crystals are irregularly-formed minerals with no crystal faces shown. These can be illustrated as massive quartz from veins and massive fluorite layers.
Let’s discuss the most common crystal habits in more detail. The following terms are most commonly used.
- Isometric forms are equally developed in all three directions in space. Garnet rhombic dodecahedrons, magnetite octahedrons, and pyrite cubes can be an example of such shapes.
- Forms elongated in one direction (prismatic, columnar, columnar, needle-like, fibrous formations). For example, crystals of amphibole, tourmaline, epidote, and asbestos.
- Forms are elongated in two directions, while the third direction remains short. These include tabular, lamellar, leafy, and scaly crystals. For example, lamellar crystals of hematite, mica, tabular chrystals of feldspar, scaly crystals of sericite, etc.
Crystals in rocks and ores rarely form well-formed crystals. They compose different aggregates, consisting of crystals of one or more minerals. The most common mineral aggregates you can find are agate, chalcedony, flint, carnelian, turquoise, malachite, and azurite (mineralogically, it is a rock, as it consists of 3 different types of minerals), hematite, and goethite.
Druses (brushes) are groups of crystals that have grown perpendicular or almost perpendicular to the surface of cracks, the wall of a vein, or a cavity in a rock. Purple variety of quartz – amethyst is probably the best-known example of druses. However, there are a lot of other minerals, like calcite and celestine, which create druses of beautiful shiny crystals.
Concretions are spherical (sometimes irregular, but rounded, flattened) aggregates of a radially radiant structure. There is often a grain in the center, which served as a seed during the growth of the concretion. Most often, they are formed in porous sedimentary rocks like sands and clays. Examples are nodules of calcite, pyrite, and phosphorites. The sizes of these formations range from millimeters to tens of centimeters, sometimes creating popular pyrite dollars.
Oolites (beans or peas) are formed when a mineral crystallizes from a solution on some grain as if covering it with shells overlapping each other (like an onion). They are characteristic of some varieties of bauxites, manganese, and iron ores (limonites). Oolites range in size from millimeters to several centimeters.
Fibrous aggregates usually form in fractures. These are veins of silky gypsum(selenite), serpentine asbestos, and columnar calcite.
Botryoidal (kidney-shaped) are smooth bulbous or globular shapes where each semi-sphere has a spherulite-like, radial-radiant structure. Typical examples are goethite and malachite. Commonly, kidney-shaped aggregates are formed in various cavities in near-surface zones of destruction and weathering of ores and rocks.
Granular aggregates are intergrowths of mineral grains, usually irregular in shape, forming a rock or ore.
Earthy masses – the smallest grains of a mineral in the form of a powder, for example, jarosite, kaolinite.
TIP: Many people use the terms rock, mineral, crystal, stone, and gemstone interchangeably, but these terms actually have specific meanings. Check out the differences between these terms in the article below:
Rock, Mineral, or Crystal? What’s the Difference?
Conclusion
Mineral properties we have just discussed above are a significant part of mineralogy. It is fundamental knowledge that is covered during two or three semesters of bachelor’s studies in university.
It is a lot of information provided; however, you don’t need to memorize it. Each time you found a stone, you can use this article as a step-by-step guide for mineral properties check.
After some time, you will know what properties should be checked first for this specific kind of mineral and what properties can be omitted.
Color is the most obvious property of a mineral, but, for sure, it should be used in a combination with other properties like streak, luster, and hardness.
The morphology of crystals is also a very important feature that should not be underestimated. Also, some minerals can shock you with some properties, like strange odor, or star-like optical phenomenon.
TIP: If you are located in the United States, and you love rockhounding, then we have comprised a list, maps, and guidelines about the best rockhounding sites near you! Find out more in the article below:
Rockhounding Near Me: Best Locations State-by-State (Map)