Black stones represent a wide group of minerals, rocks, and organic materials. Their dark coloration arises from diverse chemical compositions and formation processes, often leading to confusion for consumers and collectors. Distinguishing these materials requires understanding their unique origins, whether they are formed deep within the Earth, result from volcanic activity, or derive from biogenic processes.
Common Black Stones in Jewelry
Black Onyx is one of the most recognized black stones in commercial jewelry, a cryptocrystalline form of silica, which is a variety of chalcedony. Naturally, onyx exhibits alternating parallel bands of color, but the pure black material is often achieved through a dyeing process to enhance its uniform hue. With a hardness between 6.5 and 7 on the Mohs scale, this stone offers durability but is softer than many other popular gemstones.
Another popular choice is Black Spinel, a magnesium aluminum oxide mineral that naturally forms with a deep, uniform black color. Because of its excellent vitreous luster and high hardness, typically around 8 Mohs, it is frequently used as a durable and more affordable alternative to black diamonds. Its lack of cleavage also makes it ideal for faceting, allowing it to display a brilliant reflection of light.
For those seeking the ultimate in durability, Black Diamonds, known scientifically as carbonados, are the hardest natural substance, scoring a perfect 10 on the Mohs scale. Unlike clear diamonds, carbonados are opaque and gain their color from numerous microscopic inclusions of graphite and amorphous carbon. Alternatively, some designers utilize Melanite, a black, titanium-rich variety of Andradite Garnet, which is a calcium iron silicate mineral with a high luster and hardness.
Jet stands apart from mineral-based stones due to its organic origin as a form of fossilized wood. This material, a type of lignite coal, is notably lightweight and much softer, rating only 2.5 to 4 on the Mohs scale. Its relative softness requires careful setting, but its unique, warm luster and ease of carving have made it a traditional choice for ornamental objects and mourning jewelry.
Understanding Black Volcanic and Earth Stones
Obsidian is defined by its rapid geological formation, created when felsic lava cools so quickly that mineral crystals cannot form. This process results in a natural volcanic glass with an amorphous structure. Obsidian’s signature is its conchoidal, or shell-like, fracture, which produces extremely sharp edges, historically prized for tools.
Basalt is a widespread extrusive igneous rock that forms from the rapid cooling of low-viscosity, magnesium and iron-rich (mafic) lava near the Earth’s surface. Its characteristic dark color, ranging from dark gray to black, comes from a high concentration of minerals like pyroxene and olivine. These silicates contribute to its fine-grained texture and high density.
Black Tourmaline, or Schorl, is a complex borosilicate mineral defined by its distinct prismatic crystal structure and high iron content. It is commonly found in metamorphic and igneous rocks, crystallizing in long, striated columns that are opaque and dense black. Unlike volcanic glass, Schorl is a true mineral with a substantial hardness of 7 to 7.5 Mohs.
Shungite is another carbon-rich material, a black, metamorphic rock found primarily in Russia. It is largely composed of carbon, sometimes up to 98 percent. Shungite has a biogenic origin, believed to be metamorphosed oil shale, and is sometimes classified as a mineraloid with a unique, non-greasy texture.
How to Scientifically Identify Black Stones
The initial step in identifying an unknown black stone involves assessing its relative hardness using the Mohs scale. Hardness is a measure of resistance to scratching, which immediately separates soft organic materials, like Jet (Mohs 2.5–4), from much harder minerals like Black Spinel (Mohs 8) or Black Diamond (Mohs 10). A simple test can determine if a stone is hard enough to scratch common glass.
Luster and fracture patterns provide the next level of distinction by describing how light interacts with the surface and how the stone breaks. Obsidian exhibits a clear, glassy or vitreous luster and a characteristic conchoidal fracture due to its amorphous structure. In contrast, Jet displays a softer, waxy luster, while a faceted Black Spinel shows a brilliant, adamantine reflection.
The streak test, which involves scraping the stone across an unglazed porcelain plate, reveals the color of the mineral’s powder. While many black stones like Onyx and Spinel produce a colorless or white streak, other minerals can be identified by a distinct powder color. For instance, Hematite, a metallic black iron oxide, famously leaves a red-brown streak, a property that is often more reliable than the stone’s external color.