Obsidian is a form of volcanic glass, born from lava that cooled too rapidly to allow for the formation of mineral crystals. This quick solidification process gives the material its unique glass-like structure, setting it apart from typical crystalline rocks. Identification relies on a combination of distinctive visual traits and simple physical tests. Understanding these characteristics is the practical way to determine if a dark, glassy rock is truly obsidian.
Essential Visual Characteristics
Obsidian is immediately recognizable by its high, vitreous luster, which describes its striking, glass-like sheen. The surface often appears polished and reflective, similar to manufactured glass. This glassy texture is a direct result of the lack of crystalline structure.
The most common color is deep, jet-black, caused by tiny inclusions of iron and magnesium oxides. However, obsidian occurs in a variety of other forms. These include Mahogany obsidian with reddish-brown patches, and Snowflake obsidian speckled with white, radially clustered cristobalite crystals. More rare types, like Rainbow or Sheen obsidian, exhibit an iridescent or golden glow due to microscopic gas bubbles stretched along the lava flow layers.
The most telling visual sign is the conchoidal fracture pattern, which refers to the shell-like, curved breaks that form when the glass is struck. This fracture creates smooth, concave surfaces with sharp, curved ridges, which is characteristic of all types of glass. The resulting edge is extremely sharp, a property utilized by ancient cultures for tools and weapons.
Simple Physical Identification Tests
A practical way to confirm the identity of obsidian is by testing its relative hardness using the Mohs scale of mineral hardness. Obsidian typically registers a hardness of 5 to 5.5. This means it is hard enough to easily scratch window glass, which has a hardness of about 5.5, but it is softer than quartz, which registers a hardness of 7.
Assessing its density, or relative weight, is also useful. Obsidian is a silica-rich rock, and its density typically ranges from 2.35 to 2.6 grams per cubic centimeter. This relatively low density often makes a piece of obsidian feel lighter than other dark rocks of a similar size, such as basalt. This perception of lightness can be a subtle indicator.
The thin edge test is a reliable confirmation method that takes advantage of the fact that even opaque black obsidian is only dark due to numerous microscopic impurities. By holding a small or thin piece up to a strong light source, the edges will often reveal a slight degree of translucency. This light passing through the thin edge will show the true underlying color, which is often dark brown, gray, or sometimes green.
Common Look-alikes
Obsidian is frequently confused with man-made glass, which is chemically similar but lacks the natural formation context. A primary differentiator is that man-made glass or industrial slag often contains numerous, perfectly spherical gas bubbles or distinct flow marks. While natural obsidian can contain microscopic bubbles, it rarely has the uniform, visible bubbles found in manufactured glass.
Another common look-alike is pitchstone, which is also a type of volcanic glass but has a different chemical composition. Pitchstone contains a higher percentage of water, giving it a duller, more resinous or waxy luster instead of obsidian’s signature vitreous shine. This higher water content also causes pitchstone to be less brittle and less prone to the characteristic conchoidal fracture.
It is important to distinguish obsidian from basalt, a common dark, fine-grained volcanic rock. Basalt is composed of mineral crystals, giving it a dull, grainy, or microcrystalline texture. Obsidian, by contrast, is an amorphous glass with no crystalline structure, resulting in its smooth, mirror-like texture and predictable conchoidal fracture pattern.
Safety When Handling Obsidian
The sharpness produced by the conchoidal fracture requires specific precautions when handling obsidian. The edges of a freshly broken piece can be sharper than modern steel instruments, capable of inflicting deep cuts with minimal pressure. This sharpness is why obsidian was historically used for specialized cutting tools and is even used today in some surgical scalpels.
Specific safety measures must be followed when handling obsidian:
- Wear heavy-duty gloves when handling broken or rough pieces to protect hands from accidental cuts.
- Work should be done over a contained surface, as small, sharp fragments (flakes or slivers) can easily break off and scatter.
- Proper storage involves wrapping pieces individually in thick cloth.
- Alternatively, place pieces in separate containers to prevent chipping and protect the handler.