Rockhound’s Guide: Best Ways to Visually Identify Obsidian

Glittering black glass, fractured edges sharper than steel, formed in the fires of ancient volcanoes. Obsidian’s visual allure and hidden secrets have fascinated humankind for millennia. This natural volcanic glass holds the remarkable ability to reveal the geological fingerprints of its fiery origins and chronicle its use across civilizations. From the cutting edge of arrowheads to the precision of surgical scalpels, obsidian’s unique properties impart a practical elegance.

Yet to fully unlock obsidian’s identity requires patience and skill. Its glassy texture conceals a complex history frozen in time by sudden cooling eons ago. Only careful examination of obsidian’s distinctive characteristics enables the true capabilities of this remarkable material to come to light. Tapping into the knowledge and passion of expert collectors provides a wealth of insights to identify obsidian with confidence.

This guide delves into obsidian’s mystique by exploring its visual cues, chemical properties, and examples of its significance. For geologists, obsidian offers traces of ancient lava flows and volcanic origins. Archaeologists use obsidian to reconnect artifacts with their source regions and ownership histories. Collectors appreciate obsidian for its interactive discovery process and stunning specimens. The allure of obsidian extends across disciplines. By learning how to identify obsidian through scientific observation alongside personal experience, one can unlock the secrets of this natural wonder. Join the journey and see what stories obsidian’s glassy facade can tell.

What is Obsidian?

Obsidian is a glassy, silica-rich igneous rock formed from rapidly cooled lava, usually within volcanic lava flows and massive pyroclastic eruptions. The rapid cooling prevents crystallization, allowing the molten rock to solidify into a natural glass with a smooth, uniform texture. Obsidian consists mainly of silica (>65%), contains little to no water, and has low crystallinity. It ranges in color from black, red, and brown to green/blue. The composition gives obsidian its characteristic glossy surface and sharp, conchoidal (curved) fracture patterns. Structurally, it is an amorphous mineraloid rather than a true mineral due to the lack of an ordered internal crystal structure. Obsidian’s unique properties made it an important raw material for stone tools in ancient civilizations.

The key requirement for obsidian formation is the incredibly rapid cooling of felsic lava enriched in silica (>75% SiO2). Rhyolitic and andesitic lava flows meeting this compositional criteria can form obsidian under the right eruptive conditions. Typically, the outer surface of a lava flow chills quickly in comparison to the insulated interior. Similarly, lava ejected explosively into the air during volcanic eruptions can cool rapidly before hitting the ground. The minimal time for crystals to grow results in the signature amorphous texture. Cooling rates on the order of 1000°C per hour are typical. By contrast, slower cooling over months or years enables minerals like quartz and feldspars to crystallize, creating what is known as volcanic glass. The glassy state is not stable over geologic timescales, and older obsidian progressively devitrifies into microcrystalline aggregates.

My obsidian specimens derived primarily from Cascade volcanic arcs along the West Coast where active rhyolitic volcanism produces ideal formation environments. I’ve also collected examples from Yellowstone rhyolite flows and Apache Tears spheroidal obsidian from Arizona. Each location produces a distinct chemical fingerprint based on the original magma. Elemental analysis performed at geology labs aids source provenance studies and dates artifacts back to ancient civilizations that utilized obsidian. The long history of use in toolmaking speaks to obsidian’s remarkable physical properties.

Physical Properties of Obsidian

The most distinctive physical properties of obsidian stem from its amorphous, glassy structure.

• Color: Obsidian occurs in various colors like black, gray, brown, red, orange, green, or colorless. Color depends on the presence of impurities like iron and other dissolved elements. Mahogany obsidian gets its reddish color from oxidized iron. Intricately banded obsidian displays color zonation from changes in lava chemistry during the eruption. Rare varieties like rainbow obsidian exhibit iridescent banding when polished.
• Texture: The texture of obsidian is smooth, uniform, and glassy with a vitreous luster. Fracture surfaces appear conchoidal with curved ripple-like markings. Flow banding and spherulitic textures are sometimes visible. The glassy texture comes from the lack of an ordered crystal structure.
• Transparency and Luster: Obsidian is translucent in thin pieces and opaque in thick forms. The surface ranges from having a glassy to resinous luster. Transmitted light reveals the true color since surface lusters can be deceiving.
• Hardness and Density: It has a hardness of ~5.5 on the Mohs scale, allowing steel to scratch its surface. Softer minerals like calcite scrape off. The dense atomic structure gives obsidian a density around 2.4 – 2.6 g/cm^3. I measure specific gravity routinely to aid identification.
• Cleavage and Fracture: Obsidian lacks natural cleavage planes and fractures conchoidally. The fractures have curved surfaces with rippling or step-like patterns. The curve’s radius depends on obsidian composition and origin.

I’ve spent countless hours examining the fine details of obsidian surfaces under magnification. The curved fracture surfaces exhibit extremely smooth texture with faint lines akin to flowing water. Banding often parallels the internal flow patterns of the original lava. Distinct layers indicate changes in lava chemistry over time. Swirling fluid-like textures appear in spherulitic obsidian. No crystals are ever visible, only pure volcanic glass. The glassy facade conceals a complex history frozen in time by sudden cooling millennia ago.

Visual Identification of Obsidian

There are some key visual cues for identifying obsidian in the field:

• Examine color and transparency – Obsidian has dark colors like black, brown, gray. Thinner edges allow some light to pass through. Color zonation may be visible in banded varieties.
• Check for a glassy luster – Light reflects off obsidian surfaces like a glass bottle. The vitreous luster distinguishes it from other crystalline igneous rocks. A resinous luster appears on weathered surfaces.
• Look for smooth, uniform texture – Obsidian lacks visible crystals. The texture appears smooth and homogeneous without obvious grains. Flow banding may be discernible.
• Inspect fractures and edges – Clean, conchoidal fracture patterns with curved ripples are trademark characteristics. Check edges for sharpness.
• Test hardness using a steel knife – Obsidian has a hardness of ~5.5, allowing steel to scratch its surface. Softer minerals like calcite scrape off.
• Examine transparency – Thin fragments transmit light revealing the true interior color. Obsidian is transparent to translucent.
• Evaluate density – A density of 2.4-2.6 g/cm^3 confirms the glassy composition. I weigh samples and calculate specific gravity.
• Assess magnetic properties – Obsidian is non-magnetic unlike some black basalts. This helps rule out other minerals.
• Look for flow banding – Parallel color bands indicate changes in lava composition during formation.
• Note lack of crystals – Glassy obsidian never contains mineral crystals. Any white spots are gas bubbles called spherulites.

With practice, obsidian identification becomes second nature. The smooth texture, conchoidal fracturing, glassy luster and transparency become giveaway clues. Certain tests like the rock hammer ring test can indicate obsidian’s glassy amorphous structure from the high-pitched ringing sound. Examining a fresh break under a hand lens reveals curved fracture surfaces without cleavage planes or crystal faces. The presence of black glassy rocks with these characteristics in a volcanic region strongly indicates obsidian.

Obsidian may resemble black shale or anthracite coal but lacks the sedimentary rock layering. It can be mistaken for glassy rhyolite or scoria but obsidian is completely amorphous. Pumice and tuff are other volcanic glasses but are more porous. Obsidian commonly occurs near rhyolite lava flows and volcanic domes where lava cooled quickly. Finding obsidian shards or pebbles in an area signals the presence of an obsidian source nearby. With quick visual scans, I can now spot obsidian exposures from a distance based on the cleavage patterns and sheen. Collecting obsidian has honed my instincts for identification.

Chemical Properties of Obsidian

Chemically, obsidian consists mainly of silicon dioxide (SiO2, 75% or higher). It also contains approximately 10-15% aluminum oxide (Al2O3) along with traces of iron, magnesium, calcium, sodium, potassium, and water. Precise chemical composition varies between geological locations but always reflects a high silica content (65% or more SiO2).

Chemical tests can help characterize obsidian’s composition:

• X-ray diffraction shows the amorphous, non-crystalline structure.
• Electron microprobe analysis determines quantitative chemistry.
• Spectrographic analysis identifies minor and trace elements.
• Hydration rind measurements date the age of obsidian artifacts.
• Testing magnetic properties reveals low iron content.
• Measuring refractive index confirms the high silica glass composition.
• Specific gravity relates to the density and silica content.
• Solubility in acids depends on silica and aluminum abundance.
• Flame tests for sodium and potassium give additional compositional data.

During my geology coursework, I gained extensive hands-on experience with these laboratory techniques. Training my eye to recognize obsidian’s visual qualities prepared me to validate its identification through quantitative tests. The non-crystalline structure revealed by X-ray diffraction confirms the glassy texture. Elemental analysis provides the concrete chemistry pointing to a rhyolitic source composition. I enjoyed mapping out obsidian sources by their distinctive trace element fingerprints. The composition tells a story of how the molten rock evolved before suddenly freezing into glass.

Studying the chemical signature provides information on obsidian formation and source origins. Each volcanic glass flow has a distinct chemistry influenced by the magma composition. Chemical data allows archaeologists to trace artifacts back to their geological source. The concept of an obsidian “fingerprint” relies on subtle elemental variations that permit sourcing and dating of artifacts. Researching the quantitative properties of obsidian taught me how identification extends beyond visual qualities alone.

FAQs

What are the uses of obsidian?

Obsidian has served important uses throughout history as cutting tools, knife blades, and scrapers due to its sharp conchoidal fracturing. It was highly valued for making weapons and stone tools in ancient civilizations who used techniques like pressure flaking to shape obsidian. Today, obsidian is popular for making scalpels for delicate eye and heart surgeries. The incredibly sharp thin edges reduce tissue damage compared to steel. Obsidian is also used to make jewelry, as a decorative rock in landscaping, and in optical telescopes due to its high transparency. Artists carve obsidian into beautiful figurines and ornamental objects. Its versatility comes from the unique glassy properties. With the right approach, obsidian can be worked into a variety of forms to meet both practical and aesthetic purposes.

How is obsidian different from other volcanic glass?

Obsidian is a naturally occurring volcanic glass, but not all volcanic glass is obsidian. The difference lies in how they formed. Obsidian forms when felsic lava like rhyolite cools rapidly with minimal crystal growth. This results in its amorphous structure. Volcanic glass refers to any glass formed from quick cooling, including basaltic glass from mafic lava. Obsidian is high in silica (over 75% SiO2) while other volcanic glasses have less silica around 50% and more iron and magnesium. Pumice and scoria are more porous volcanic glasses. Obsidian cools the fastest and thus appears glassier. Compositionally, obsidian is a high-silica volcanic glass meeting specific formation criteria.

Can obsidian be used for jewelry?

Yes, obsidian makes a beautiful natural gemstone for jewelry purposes. Obsidian has a glassy luster that takes an excellent polish. It is usually cut en cabochon to form rounded stones that enhance its smooth glossy texture. Certain varieties like rainbow obsidian, silver sheen obsidian, and snowflake obsidian display attractive banding patterns when cut. Obsidian has a hardness around 5.5, so it’s durable enough for most jewelry pieces but can gradually scratch over time with rough wear. Quality obsidian jewelry makes a unique addition to anyone’s collection. As an amateur lapidary, I’ve crafted my own obsidian pendants which draw admiring looks for their deep glossy black surfaces and fiery rainbow highlights. With care, obsidian jewelry can last a lifetime.

How can I tell if obsidian is real or fake?

To identify real obsidian, look for a smooth glassy texture without air bubbles or layered patterns. Fakes like black glass will appear too perfect and lack natural rippling. Examine the fracture’s curvature and test hardness using a steel point. Real obsidian will fracture conchoidally and show evidence of scratching. Check for a vitreous luster and light translucency on thin edges. Perform specific gravity measurements to confirm around 2.4-2.6 g/cm^3 density. Try exposing it to strong light to look for any rainbow sheen. The composition should be non-magnetic. Advanced tests like chemical analysis or X-ray diffraction can definitively determine if a sample is true volcanic glass. With practice, developing an attentive eye is key to spotting fake obsidian. Trust your instincts if something about the texture, luster or feel seems artificially manufactured.

What are the dangers of handling obsidian?

Fresh obsidian fractures can produce razor-sharp edges that can easily cut skin. Wear protective gloves when handling raw obsidian samples. The fine shards present health hazards if inhaled. Always wash hands after working with obsidian and operate on a stable surface to avoid risks. When used correctly, obsidian is safe for cutting tools and jewelry but proper precautions are advised. The same properties that give obsidian its unique benefits also make it potentially dangerous. With adequate care, the dangers can be minimized. I suffered a few minor cuts early on before learning just how sharp obsidian edges can be. Now I know to control my excitement and handle specimens gently to avoid accidental injury. A healthy respect for obsidian helps mitigate hazards. By using sound judgment, even novice collectors can enjoy obsidian safely.

Conclusion

In summary, obsidian has distinct visual properties including a glassy luster, conchoidal fracturing, smooth uniform texture, and dark transparent coloration. Combining basic visual tests with an understanding of its chemical composition allows for confident identification of this unique volcanic glass. Geologists value obsidian for the clues it provides into past volcanic processes and artifacts reveal ancient civilizations. For the hobbyist, obsidian sparks fascination with its glossy facade and myriad formation patterns. Yet the appeal also comes from mindfulness of its potential hazards requiring prudent handling.

Through experience gained over years of collecting obsidian across volcanic terrains, I’ve developed an intuition for recognizing its telltale characteristics. That initial thrill when spotting a fractured chunk of rainbow-flecked obsidian eroding from a slope still strikes me today. Obsidian forever connects me to the forces that shaped the land, and its glassy texture represents untold stories frozen in time. While hard scientific data helps characterize obsidian, the human passion for discovery drives deeper understanding. Combining quantitative analysis with qualitative intuition forged through personal experience creates a rewarding relationship with this most captivating volcanic rock. Obsidian harbors many secrets, for those patient enough to reveal its true identity.