Basic Rock & Mineral Identification
- Chelsea Hoffland
- Sep 29, 2022
- 4 min read
Updated: Oct 11, 2022
Recognize basic defining features of crystals, rocks, & minerals with no specialty equipment
Identifying your crystals can be a difficult process, especially if you're not sure where to start. Below I've laid out the most basic concepts of rock identification, and from there you can cross reference with the identification chart found within each crystal post I create.
In this Article:
Color and Pattern
Color is the most basic way to identify a mineral, however it can also be highly circumstantial. Many rocks and minerals portray multiple colors, or at the very least, different shades of the same color. That, along with the fact that you can have hundreds of minerals of the same color, and there's no way to tell for certain. It is very rare for a rock to be identified strictly on the color pattern.
Transparency
While color may not be the most reliable source of identification, the transparency of a specimen will be a lot more helpful. Transparency refers to the ability of a specimen to transmit light and is usually defined as one of three words; opaque, translucent, and transparent. Hold a flashlight under/behind a stone and see how much light it lets through.
Transparency Terms
Opaque: no light passes through Translucent: light is diffused, meaning it passes through but is not clear Transparent: images are seen clearly through the specimen
Streak
The streak of a rock is the color it portrays when being scraped across a streak plate (a tile of unglazed ceramic). If there are large swatches of different minerals within the rock, this color may vary depending on which part of the rock is tested. This color will also not always match the color of the rock itself. For example, hematite leaves a red streak and fluorite always leaves a white streak.
The reason for this difference is simply due to the presence of impurities. Impurities within the rock itself will absorb certain wavelengths of light, drastically changing the colors that the specimen itself reflects. Once this specimen is dragged across a streak plate however, these impurities break down and no longer have a significant impact on the absorption of light.
Hardness
Most of us know that diamonds are one of the hardest materials known to man, but what does that mean? With that question in mind, we turn to the Moh's Hardness Scale. This scale is a chart of relative hardness, from diamonds all the way down to talc. It was created in 1822 by Frederich Mohs and is used to compare the hardness of different minerals. The catch is, some minerals present different hardness depending on which direction along their crystal structure they are tested (one example being kyanite).
To test hardness you need a known material along with the material you are testing. A softer substance (lower number on the scale) will not scratch, and will be scratched by, a substance of a higher hardness. Firmly drag one across the other, then gently wipe away any dust that appears. If there is a mark, the stationary specimen is softer. If not, the one you used to scratch with is softer. Sometimes it can be difficult to tell. In this case, they could have very similar hardnesses.
I recommend having a piece of known quartz (7 Hardness), glass/obsidian (5.5 H), and a natural finger nail (2.5 H) at the very least. This will help you significantly narrow down your search when looking into unknown specimens.
Moh's Scale of Relative Hardness
10: Diamond 9: Corundum 8: Topaz 7: Quartz 6: Feldspar 5: Apatite 4: Fluorite 3: Calcite 2: Gypsum 1: Talc
Lustre / Luster
Lustre is defined as the way that light interacts with the surface of a specimen. Generally it can be difficult to observe in a photo, and is much better identified through videos or first-hand site. It is also important to note that this is slightly subjective, and can also be altered through sanding and polishing. The best way to observe lustre is in person on a freshly chipped area of the specimen.
Common Terms Defining Lustre:
Metallic: highly reflective, shines like metal Vitreous/Glassy: glossy or shiny like a sheet of glass Waxy: appears to be covered in wax, picture a smooth candle Dull/Earthy: little to no lustre, matte (think unglazed pottery) Greasy: appear to be covered in grease, dull sheen Pearly: iridescent like a pearl, grows in thin transparent co-planar sheets Resinous: appear to be coated in resin, similar to that of plastic Silky or Fibrous: a fibrous sheen that is reminiscent of silk
Optical Phenomena
Optical phenomena are slightly more unique to each stone and are represented by a play in light within the stone. Most of them are quite easy to identify:
Optical Phenomena Examples
Schiller/Labradorescence: a display of color below the surface as light reflects between layers (often seen in feldspars, particularly labradorite) Color Change: colors change appearance based on lighting (alexandrite & sapphire) Pleochroism: presents different colors depending on the angle (iolite & alexandrite) Chatoyancy: tightly packed parallel fibers appear to move as the specimen does (tigers eye & charoite) Aventurescence: names after aventurine, appears as sparkly reflections within the stone (aventurine & goldstone) Asterism: a display of a luminous star shape (spinel & rubies) Iridescence: often referred to as "fire" similar to oil on water (precious opal)
Refractive Index
The refractive index of a mineral is its ability to bend light. It basically determines how much light is bent (or refracted) when it passes through the crystal. Refractive index is typically reported as a small, decimal number and is measured using a refractometer.
Specific Gravity
The specific gravity of a specimen is the ratio of its mass vs. the mass of an equal volume of water. It can vary slightly due to impurities within the stone and is generally quoted as a small range.
For example: The specific gravity of quartz is about 2.65. This means 1 cubic centimeter of quartz is 2.65 times heavier than 1 cubic centimeter of water.
Cleavage
Cleavage describes how a specimen will break along certain cleavage planes where its bonds are weakest. These planes are defined as 2-dimmensional surfaces and cause smooth edges along the break. Cleavage is defined both by the direction and the quality of the cleaves.
Quality of Cleavage
Perfect: causes a completely flat, lustrous surface
Imperfect: breaks in planes but they are not smooth
Poor: still breaks in planes but is extremely rough
None/Nonexistent: No cleavage. See "Fracture" below
Direction of Cleavage
Basal/Pinacoidal/Planar: 1 cleavage plane causing flat sheets (mica)
Prismatic: 2 planes, rectangular if at 90 degree intersect, parallelogram if not at 90 degrees
Cubic: 3 planes intersect at 90 degrees causing cubes (halite)
Rhombohedral: 3 planes not at 90 degrees (calcite)
Octahedral: 4 planes causing an 8-sided shape (fluorite)
Dedecahedral: 6 planes causing a 12-sided shape (sphalerite)
Fracture
Fracture happens when there is no cleavage along a break. The following are examples of common fracture types.
Types of Fracture
Conchoidal: smooth curve
Even: rough but fairly plane
Uneven: irregular & rough
Hackly: sharp & jagged
Splintery/Fibrous: breaks into fibers or splinters
Crystal System
Crystals are classified into 7 different crystal systems based on a combination of their relative length, their axes, and the angle at which their axes meet. This can get pretty in-depth, but it's important to understand the "unit cell," which is the most basic repeating unit that has full symmetry to the crystal structure.
The Seven Crystal Systems
Cubic (Isometric): 4 threefold axes of rotation (unit cell forms a cube)
Hexagonal: 1 sixfold axis of rotation
Trigonal: 1 threefold axis of rotation
Tetragonal: 1 fourfold axis of rotation
Orthorhombic: 3 twofold axes of rotation or 1 twofold axis of rotation and 2 mirror planes
Monoclinic: 1 twofold axis of rotation or 1 mirror plane
Triclinic: no symmetries
*crystal system information copied directly from Wikipedia to keep it simple. https://en.wikipedia.org/wiki/Crystal_system
Crystal Habit & Form
The crystal habit of a specimen is the tendency of a mineral to grow in a specific shape. It is largely dependent upon the arrangement of atoms within the mineral (known as the crystal lattice) however it can also be affected by variables within the environment surrounding the stone (i.e. space limitations). The actual resulting shape of a crystal is often referred to as its "form".
Common Crystal Habits
Prismatic: elongated crystals that are longer than they are wide (tourmaline & hornblende)
Tabular: plate-like & flat (feldspar)
Acicular: needle-like shape that tapers down (rutile)
Fibrous: grows in fine, fiber-like formations (serpentine)
Cryptocrystalline: microscopic sized crystals (jasper & agate)
Massive: too large to distinguish crystal form
Granular: obvious mineral grains (olivine)
Bladed: a collection of elongated crystals similar to knife blades (kyanite & gypsum)
Cubic: 6 square faces (pyrite & fluorite)
Radiating: grow outwards from a central point (kyanite)
Botryoidal (globular): grow in a rounded shape (malachite & grape agate)
Drusy: cluster of very small crystals (quartz)
Dendritic: a vein-like branching pattern (dendritic agate)
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