Black tourmaline (Schorl) is the most common species within the complex tourmaline mineral group. This iron-rich borosilicate is a widespread mineral prized for its deep, opaque black color. Identifying this mineral requires a systematic approach, moving from visual characteristics to physical tests and finally to its unique crystalline properties.
Fundamental Visual Characteristics
Black tourmaline is consistently opaque black, sometimes displaying a very dark bluish or brownish hue under intense light. It typically exhibits a vitreous to resinous luster, giving it a glassy sheen. The mineral frequently forms elongated, prismatic crystals recognizable by their unique cross-section. This shape is distinctly trigonal, appearing as a rounded triangular outline when viewed end-on.
The defining visual feature of a raw black tourmaline crystal is the presence of parallel lines or grooves, called striations, which run vertically along the prismatic faces. These vertical striations are a strong indicator of the tourmaline structure and are often visible even on tumbled or polished pieces. Black tourmaline lacks distinct cleavage. Instead, a fresh break will show an uneven or conchoidal fracture pattern.
Simple Physical Testing Methods
The Mohs scale provides a reliable way to differentiate black tourmaline from softer materials. Black tourmaline registers between 7 and 7.5, making it harder than common glass (rated approximately 5.5). A simple scratch test involves attempting to draw a line on glass with the mineral; genuine black tourmaline should leave a scratch, while a softer imitation will not.
A streak test determines the color of a mineral’s powder when rubbed across an unglazed porcelain plate. Despite its deep black color, black tourmaline produces a white or colorless streak, which aids identification. Black tourmaline also has a relatively high specific gravity, typically ranging from 3.06 to 3.20. This density is higher than many common imitations like glass or plastic, meaning the stone will feel noticeably heavy for its size.
Distinguishing from Common Look-Alikes
Black tourmaline is often confused with other black stones, but its characteristic features allow for clear differentiation. Black obsidian, which is volcanic glass, is amorphous and lacks crystalline structure, meaning it will not exhibit the vertical striations found on tourmaline. Obsidian is also significantly softer (Mohs hardness of about 5 to 5.5) and displays a very smooth, conchoidal fracture.
Jet, a form of fossilized wood, is far softer than tourmaline (Mohs hardness of only 2.5 to 4). Jet feels distinctly warm to the touch and has a low density, whereas tourmaline feels cool and is substantially heavier. Black onyx, a variety of chalcedony, is a microcrystalline quartz with a Mohs hardness around 6.5 to 7. Onyx typically lacks the prismatic crystal habit and vertical striations that characterize black tourmaline. Onyx and black agate often show parallel banding, a feature absent in solid black tourmaline.
Unique Crystal Properties
Black tourmaline’s internal structure dictates its fracture pattern, described as uneven or small conchoidal. This reflects a splintery or shell-like break rather than smooth cleavage planes, contrasting with the perfect cleavage seen in other minerals. The mineral’s complex crystalline structure gives rise to its unique electrical behaviors.
Black tourmaline exhibits both pyroelectricity and piezoelectricity. Pyroelectricity means the crystal develops a temporary static electrical charge when heated. Piezoelectricity is the ability to generate an electric charge when subjected to mechanical stress, such as pressure or rubbing. These electrical properties, arising from the crystal’s asymmetrical internal structure, serve as an advanced method for confirming identity.