The word mineral comes from the Latin minera, or mine. Until the nineteenth century, minerals were simply things that came out of mines—things that had economic value, like gold, various ores, and coal.
The modern definition is more specific. A mineral is a homogeneous, naturally occurring inorganic substance with an orderly atomic structure
Every mineral has distinct physical properties. It usually takes the combination of properties to identify a mineral (Hurlbut 1971; Klein and Dutrow 2007). On the other hand, if you’ve seen enough minerals, you can tell not only the mineral but even where it is from. Amethyst in a greenish volcanic matrix is almost always from Minas Gerais, Brazil:
Large crystals of rhodochrosite are almost always from the Sweet Home Mine near Alma, Colorado:
The best dioptase crystals are from Tsumeb, Namibia:
Barite roses come from the Garber Sandstone near Noble, Oklahoma:
And classic quartz crystal clusters are from Hot Springs and Mount Ida, Arkansas:
Physical Properties of the Minerals
Color may be the most obvious aspect of a mineral, but it can be misleading because several minerals can have the same color, and one mineral can have several colors. In addition to color in sunlight, some minerals display luminescence: they emit light of a different color. These minerals fluoresce or glow various colors under ultraviolet light (e.g., fluorite, calcite, scheelite). Some specimens of a mineral may fluoresce whereas other specimens of the same mineral do not.
Streak is the color of a powdered mineral. To determine streak, one scratches a white porcelain plate with the mineral. The streak is frequently different from the color of the crystalline mineral.
Luster refers to the way a mineral’s surface reflects light. There are two main types of luster, metallic and nonmetallic. Pyrite, gold, copper, and galena have a metallic luster.
Nonmetallic luster is subdivided into adamantine (diamond, corundum), vitreous or glassy (quartz), pearly or waxy (biotite, quartz), silky or fibrous (asbestos), greasy (chrysocolla, smithsonite), and dull or earthy (kaolinite). Luster can be quite subjective, with different categories grading into one another. Some minerals, such as quartz, can also exhibit more than one type of luster.
Transparency refers to the amount of light that can pass through a mineral. Some minerals are transparent (diamond, quartz), others are translucent (calcite, quartz, muscovite mica), and yet others are opaque (feldspar, biotite mica, quartz). Transparency alone is not diagnostic because the same mineral can fall into more than one category.
Cleavage is the tendency of a mineral to break along the planes of weakness in its crystal structure. These weaknesses are parallel to layers of atoms in the crystal that form lat, repeating surfaces. The bonds between atoms in a layer are stronger than the bonds between the layers. There are six kinds of cleavage:
• Basal (or pinacoidal), which is parallel to the base of a crystal (mica)
• Cubic, where a mineral cleaves parallel to the faces of a cube (halite)
• Octahedral, where a mineral will cleave parallel to one of eight crystal faces (diamond, fluorite)
• Dodecahedral, where a mineral will cleave parallel to 1 of 12 crystal faces (sphalerite)
• Rhombohedral, where a mineral cleaves parallel to the faces of a rhombohedron (calcite)
• Prismatic, where a mineral cleaves parallel to a vertical prism (lazulite, zircon)
Cleavage occurs in all minerals to some degree. Micas have well-developed cleavage that results in smooth, thin sheets. Quartz, on the other hand, exhibits cleavage only when the crystal is heated and then plunged into cold water.
Fracture refers to breakage that does not occur along cleavage planes. In some minerals, the bonds between atoms are equally strong throughout the crystal. When these minerals break, you get an irregular surface or fracture. The geometry of this surface can help you identify the mineral. Fractures can be conchoidal (“clam-shell,” as seen in lint and obsidian), hackly or jagged (as seen in rough native metals like copper), fibrous or splintery (as seen in asbestos), uneven (a rough, irregular surface), and earthy.
Hardness is the resistance of a mineral’s surface to scratching. The most common way to gauge hardness is by using the Mohs scale, which ranges from 1 to 10. The Mohs scale is strictly relative. For example, calcite (with a Mohs value of 3) is harder than gypsum (with a Mohs value of 2), but you can’t say that calcite is twice as hard as gypsum.
Hardness refers only to a mineral’s resistance to scratching—not its resistance to breaking.
Diamond, which has the highest Mohs value, will still shatter if you smash it with a hammer.
Specific gravity is a measure of mineral density. It is the mineral weight relative to water, which has a specific gravity of 1.0 at 4°C. If a mineral such as quartz has a specific gravity of 2.6, then it is 2.6 times the weight of an equal volume of water.
Special Powers of Crystals
Do crystals have special powers? Yes! Crystals possess certain special properties, such as the ability to vibrate at a constant frequency, which make them useful in technology.
Quartz crystals, for example, are used to keep time in watches. When doped with special impurities, crystals can also act as transistors in radios. Some crystals produce an electrical charge when compressed, making them useful in precision instruments. Crystals are also aesthetically pleasing (in other words, pretty).
Can crystal power be harnessed for emotional healing, channelling positive energy, balancing internal yin and yang, seeing the future, or warding off harmful rays? There is no evidence to support any such claims. However, in some places, crystals do seem to have a strange and potent power to generate New Age tourism.
By G.L. Prost and B.P. Prost