Black stones come in many different types, each with their own unique properties and origins. In this article, we will explore some of the most common black gemstones and minerals, looking at where they come from, how they form, and what makes them black.
Obsidian
Obsidian is a naturally occurring volcanic glass that forms when lava from volcanoes cools rapidly. The rapid cooling prevents crystals from forming, resulting in a hard, glassy texture. Obsidian can form in a variety of colors, but black obsidian is one of the most common.
Black obsidian forms when the lava is high in iron and magnesium. As the molten rock cools, these elements contribute to the dark coloration. In some cases, black streaks or spots known as inclusions may form within obsidian as well. These inclusions are made up of different minerals like magnetite, creating black pigmentation.
Obsidian is found in many locations worldwide where volcanic activity has occurred. Major sources include the United States, Mexico, Greece, and Iceland. Archaeologists have also found obsidian artifacts dating back thousands of years from ancient cultures who used the material to craft tools and weapons.
Onyx
Onyx is a banded variety of chalcedony, a cryptocrystalline form of quartz. It has parallel bands of alternating black and white layers. The black bands are made up of iron oxide or hornblende, while the white layers are composed of pure quartz or feldspar.
Onyx forms when silica-rich solutions permeate sedimentary rocks composed mainly of limestone or shale. As the minerals crystallize and solidify, they create the distinctive bands. Onyx can also form when limestone caves fill with silica-rich water, creating layers on the cave walls.
Major sources of onyx include Brazil, Mexico, China, India, and the United States. Onyx has been used ornamentally and for engraving since ancient times. When polished, the alternating bands create an eye-catching pattern.
Black Tourmaline
Tourmaline refers to a group of complex silicate minerals that form in long prisms with rounded triangular cross-sections. The most common type of black tourmaline is known as schorl.
Schorl tourmaline gets its dark color from high quantities of iron. As schorl crystals form deep underground in igneous rocks, they incorporate iron into their molecular structure which causes the black hue. Major sources of black tourmaline include Brazil, Pakistan, Africa, and Maine in the United States.
In addition to its use as a gemstone, black tourmaline has unique pyroelectric and piezoelectric properties. Pyroelectricity means it can generate an electrical charge when heated or cooled. Piezoelectricity means it accumulates a charge when mechanically stressed. These traits make tourmaline useful for pressure gauges, sonar devices, and electrical components.
Black Agate
Agate is a variety of chalcedony characterized by distinctive banded patterns and colors. Black agate specifically forms when trace amounts of iron are present in the silica solution as the agate crystallizes. The iron imparts a grayish-black color to the mineral.
Like other agates, black agate begins forming when silica gels precipitate into openings and cracks in host rocks. Repeated crystallizations of silica create the layered, banded patterning. Black agate typically has botryoidal banding, which means the layers form in grape-like clusters.
Major sources of black agate include Africa, Brazil, India, and the United States. When polished, black agate has a glossy surface with distinctive veining. It has been used ornamentally since ancient times and is also carved into cameos.
Black Jasper
Jasper is a variety of chalcedony similar to agate but with a grainier, less organized structure. It also derives its color from mineral impurities. Black jasper can form when iron, manganese oxides, or carbon are present as jasper crystallizes.
Black jasper has an opaque appearance and dull to vitreous luster. It forms through precipitation of silica-rich fluids in fractures, veins, between layers of rocks, and in cavities. Major sources include Egypt, Madagascar, Russia, and the western United States.
In metaphysical circles, black jasper is believed to empower stability and balance. It’s also thought to absorb negative energy. Historically, black jasper was used to make bow drills in ancient Egypt.
Hematite
Hematite is not a single mineral but rather a grouping of iron oxide minerals with the same crystallization habits. The iron oxide makes hematite opaque with a steel gray to black color. When found in nature, hematite forms as botryoidal clusters, reniform masses, tabular crystals, and more.
Hematite mainly forms through precipitation from hot aqueous solutions enriched with iron. This can occur through volcanic processes, hydrothermal changes, and metamorphic reactions. Major hematite deposits have been found in Brazil, Australia, and parts of the United States.
Beyond its use as a pigment, hematite is also an important iron ore. It’s one of the primary industrial minerals used in steel production and many other applications.
Magnetite
Magnetite is a black oxide mineral belonging to the spinel group. It has the chemical formula Fe3O4 and is known for its strong magnetism. In fact, magnetite is the most magnetic naturally occurring mineral.
Magnetite forms through a process known as magmatic segregation in igneous rocks. As magma cools, magnetite and other minerals crystallize out, concentrating iron oxides. Major deposits are located in Australia, Sweden, and parts of the eastern United States.
In addition to general industrial uses, magnetite has special applications that rely on its magnetic properties. These include medicine, particle physics research, manufacturing, and nanotechnology.
Lava Stones
Lava stones or lava rocks refer to a variety of igneous rocks formed from volcanic lava flows that have hardened. They are often porous and vesicular because of gas bubbles in the lava. When lava cools rapidly, it can form black, glassy rocks like basalt and obsidian. Slower cooling forms coarser-grained gabbro and dolerite.
Lava stones occur anywhere volcanism has recently been active. Major locations include Hawaii, Italy, Iceland, and Central America. Due to their porous nature, lava stones are sometimes used in landscaping, horticulture, and spa treatments. The stones can gradually release moisture to the surrounding soil or during hot stone massages.
| Stone | Composition | Origins | Properties |
|---|---|---|---|
| Obsidian | Volcanic glass | Formed from rapidly cooled lava that is high in iron and magnesium. Major sources include the US, Mexico, Greece, and Iceland. | Hard, glassy texture. Can contain black streaks from mineral inclusions. |
| Onyx | Banded chalcedony | Forms when silica solutions fill limestone caves and sediments. Major sources include Brazil, Mexico, China, and India. | Parallel black and white banding due to alternating layers of quartz/feldspar and iron oxides/amphiboles. |
| Black Tourmaline | Schorl variety of tourmaline | Forms deep underground in igneous rocks where it incorporates iron, causing black color. Major sources include Brazil, Africa, Pakistan, and Maine. | Pyroelectric and piezoelectric properties make it useful for electrical and mechanical applications. |
| Black Agate | Variety of banded chalcedony | Forms when silica gels fill cavities in host rocks. Iron imparts black color. Major sources are Africa, Brazil, India, and the US. | Distinctive botryoidal banding and high polish. |
| Black Jasper | Opaque, granular chalcedony | Precipitates from silica solutions in fractures and cavities. Major sources include Egypt, Madagascar, Russia, and the western US. | Believed to impart stability and absorb negative energy in metaphysical circles. |
| Hematite | Iron oxide mineral group | Forms through precipitation from iron-rich aqueous solutions. Major deposits located in Brazil, Australia, and the US. | Important iron ore mineral used in steel production. |
| Magnetite | Iron oxide mineral | Crystallizes through magmatic segregation in igneous rocks. Major deposits in Australia, Sweden, and the eastern US. | Known for strong magnetism with special applications relying on magnetic properties. |
| Lava stones | Igneous rocks | Formed from volcanic lava flows. Major locations include Hawaii, Italy, Iceland, and Central America. | Porous, vesicular nature makes useful in horticulture and spa treatments. |
Conclusion
In summary, there are many diverse black stones that originate through various geological processes. Volcanic activity often plays a key role in creating black obsidian, tourmaline, hematite, magnetite, and lava rocks. Meanwhile, black agate, onyx, and jasper form through precipitation of silica solutions into cavities and rocks. The common thread among most black stones is the incorporation of iron during crystallization, which imparts the dark color.
Black stones have served important cultural purposes throughout human history as tools, decorative objects, and metaphysical artifacts. Their unique properties also lend them to special modern applications, like hematite’s use in steel making. As geology continues to shape the Earth, new black stone deposits will likely form and be discovered around the globe.
