There are a few different types of rocks that can appear purple in color. The most common purple rocks are amethyst, fluorite, lepidolite, and charoite. The purple color in these rocks is caused by trace impurities and mineral structures that reflect light in the violet and purple wavelengths. Determining the exact type of purple rock requires careful examination of properties like hardness, cleavage, streak, luster, and other identifying characteristics. This article will provide an overview of some of the main purple rocks, what makes them appear purple, where they can be found, and how to identify them.
Amethyst
Amethyst is a variety of crystalline quartz that owes its violet to purple color to inclusions of iron impurities within the crystal structure. The iron ions produce strong absorption of yellow and blue light, leaving the complementary color of purple to be transmitted and observed. Amethyst rates 7 on the Mohs hardness scale and has a trigonal crystal structure. It is found in geodes and alluvial deposits across the world, with major sources including Brazil, Uruguay, Russia, India, and North America. Distinctive properties of amethyst include:
| Hardness | 7 |
| Color | Various shades of purple |
| Luster | Vitreous |
| Streak | White |
| Cleavage | None |
| Fracture | Conchoidal |
Amethyst is valued as a gemstone and carved into ornamental objects and jewelry. It has long been associated with royalty, luxury, sobriety, and tranquility. The largest deposits of high quality amethyst are found in Brazil. Other major sources include Mexico, Canada, Russia, India, Sri Lanka, and the United States.
Fluorite
Fluorite is a halide mineral composed of calcium fluoride. Trace impurities of yttrium or other rare earth elements can produce purple varieties of fluorite. The purple color results from electronic transitions associated with these impurity elements. Fluorite has a cubic crystal structure and perfect cleavage in four directions. It registers 4 on the Mohs hardness scale. Some key traits of purple fluorite include:
| Hardness | 4 |
| Color | Various shades of purple |
| Luster | Glassy |
| Streak | White |
| Cleavage | Perfect in 4 directions |
| Fracture | Sub-conchoidal to uneven |
Fluorite occurs in veins in sedimentary, igneous, and metamorphic rocks globally. Notable deposits of purple fluorite have been found in the Hunan province of China, certain localities in Mexico, New Hampshire and Illinois in the United States, and Bavaria in Germany. Fluorite is used for ornamental carvings, optics, lapidary works, flux in metal smelting, and fluoride chemicals. The vivid purple color varieties are popular for mineral specimens and jewelry.
Lepidolite
Lepidolite is a lithium-rich mica mineral that is typically rose, violet, or purple in coloration. The purple hues are produced by manganese substitutions within the mineral’s structure. Lepidolite has a monoclinic crystal system and perfect basal cleavage. It has a hardness of 2.5 to 4 on Mohs scale. Some distinctive traits include:
| Hardness | 2.5 – 4 |
| Color | Rose to purple |
| Luster | Pearly to vitreous |
| Streak | White |
| Cleavage | Perfect basal |
| Fracture | Uneven |
Lepidolite occurs in lithium-bearing pegmatites, often accompanying minerals like tourmaline, quartz, and feldspar. Notable occurrences are in Brazil, Canada, Sweden, and the United States. Lepidolite is valued as an ore of lithium and used ornamentally in crystal healing traditions. The purple color varieties are prized by collectors.
Charoite
Charoite is a rare silicate mineral named for the Chara River area of Siberia where it was discovered. Trace elements like manganese give charoite its vivid purple coloration. It has a monoclinic crystal structure and a hardness of 4.5 to 5 on Mohs scale. Some identifying traits are:
| Hardness | 4.5 – 5 |
| Color | Purple, violet, lilac |
| Luster | Vitreous to pearly |
| Streak | White |
| Cleavage | Indistinct |
| Fracture | Conchoidal to uneven |
Charoite forms a unique fibrous intergrowth with minerals like quartz and potassic feldspar. It is found exclusively along the Chara River area in eastern Siberia. Charoite is used as a decorative stone in carvings, cabochons, and other ornamental objects. Its rare purple color and chatoyancy make it a sought after collector’s stone.
Identifying Purple Rocks
Identifying the exact purple rock requires inspecting certain diagnostic properties in addition to color:
Hardness: Performing a scratch test against reference minerals can help pinpoint hardness on the Mohs scale. Amethyst will scratch fluorite. Lepidolite can be scratched by a knife blade.
Luster: The surface light reflection – from glassy in fluorite to pearly in lepidolite.
Cleavage and Fracture: Lepidolite has perfect basal cleavage while amethyst and charoite lack good cleavage. Fracture patterns differ between the minerals.
Streak: The powdered streak tends to be white in all the purple minerals discussed.
Environment: Understanding the geological setting and associated minerals aids identification. For example, charoite only occurs in Siberia.
Advanced analyses using optical mineralogy, chemical assays, or spectroscopy can definitively characterize purple mineral specimens. But basic physical tests provide plenty of discriminating data for most collectors.
Conclusions
The most prominent purple rocks and minerals are amethyst, fluorite, lepidolite, and charoite. Trace elements like iron, manganese, and rare earth metals produce the characteristic purple hues via absorption and transmission of certain wavelengths of light. Identifying features like hardness, cleavage, luster, and geologic setting help distinguish the purple minerals. Amethyst serves as a popular gemstone while fluorite, lepidolite, and charoite are prized by collectors for their striking purple color. Understanding what makes purple rocks purple as well as their unique mineral traits allows geology enthusiasts to more deeply appreciate these vivid examples from nature’s palette.
