🪨 FREE ROCKHOUNDING GUIDE #2

Clear Lake Obsidian Country

Lake County, California

Guide type: Geology, identification, ethical scouting, and legal collecting
Best for: Obsidian enthusiasts, volcanic-rock collectors, families, and geology-curious adventurers
Important: Clear Lake has exceptional obsidian, but it does not currently have a clearly designated public obsidian-collecting site comparable to an established rockhound area.

⚠️ Important Location Update

Clear Lake’s best-known obsidian sources are associated with Borax Lake and Mount Konocti. These areas overlap private property, culturally significant Indigenous landscapes, and archaeological sites containing thousands of years of human history.

This guide intentionally does not publish quarry coordinates or direct visitors onto Borax Lake archaeological ground. Ancient flakes, cores, projectile points, and worked obsidian are artifacts, not rockhounding souvenirs. Archaeological material found on federal land must be left exactly where it lies.

The goal is to understand Clear Lake obsidian, recognize naturally transported pieces, and collect only where land ownership and regulations clearly permit it.

📍 Where Is It?

Clear Lake lies in Lake County within the Clear Lake Volcanic Field, approximately 90 miles north of San Francisco. The volcanic field includes Clearlake, Lower Lake, Kelseyville, Lakeport, Lucerne, Mount Konocti, Borax Lake, and portions of the surrounding hills.

Use Lower Lake, California as the practical navigation hub for the southeastern side of the lake.

Geology and history stop

Anderson Marsh State Historic Park
Located along Highway 53 between Lower Lake and Clearlake.

This is an excellent place to understand the lake basin, marsh environment, and Indigenous history, but it is a look-and-learn stop, not a collecting stop. The park protects archaeological sites and a cultural landscape connected to the Koi people. California State Park regulations generally prohibit disturbing or removing rocks, minerals, gravel, or archaeological material unless a unit is specifically designated for rockhounding.

As of July 2026:

• Open sunrise to sunset
• Vehicle day-use fee listed as $4
• Dogs are not permitted
• Temporary fire restrictions may be active

Potential legal scouting area

Knoxville Management Area

This large BLM-managed area lies southeast of Clear Lake and can be approached using Morgan Valley Road. The BLM map shows public parcels intermingled with non-BLM property, so an offline land-ownership map and active GPS are essential. The area contains rugged roads, former mining country, volcanic-related geology, and substantial private-property boundaries.

Obsidian is not guaranteed at Knoxville. Treat it as a public-land volcanic-rock scouting area rather than a proven obsidian bed.

Before collecting, contact the BLM Ukiah Field Office at 707-468-4000 and ask:

“Is casual collection of naturally occurring obsidian currently permitted on BLM parcels in the Knoxville Management Area, and are there any closed areas, active claims, cultural-resource restrictions, or fire restrictions I should know about?”

That phone call is the difference between rockhounding and accidentally starring in CSI: Volcanic Glass Unit.

🚫 Places That Are Not Collecting Locations

Do not collect from:

• Borax Lake archaeological property
• Anderson Marsh State Historic Park
• Mount Konocti preserves, parks, or private parcels without written permission
• Vineyards, ranches, orchards, or vacant-looking private land
• Active mining claims
• Developed recreation sites
• Mine dumps, tailings, tunnels, adits, or remediation areas
• Locations containing worked flakes, projectile points, cores, grinding features, or concentrations of obsidian debris

A fence is not required for land to be private, and a missing sign is not permission.

🌋 The Geology

Clear Lake is not merely sitting near an old volcano. It occupies the middle of a broad, geologically active volcanic system.

The Clear Lake Volcanics range from approximately 2.1 million years old to about 8,500 years old. Their compositions range from basalt to rhyolite, although rhyodacite is the field’s most voluminous rock type. Volcanism has become increasingly concentrated south and east of the lake during the past million years.

Mount Konocti is a predominantly dacitic volcanic complex approximately 300,000 years old. The youngest known eruptions around Clear Lake occurred roughly 8,500 to 13,500 years ago, producing maar craters, ash, volcanic bombs, and other explosive deposits around the southeastern lake basin.

How the obsidian formed

Obsidian forms when silica-rich lava cools without allowing enough time for an orderly crystal structure to develop. Instead of becoming visibly crystalline rhyolite, part of the lava freezes into volcanic glass.

Clear Lake contains two especially important geochemical obsidian sources:

Borax Lake obsidian
Associated with a rhyolitic flow approximately 90,000 years old. It is commonly dark gray to black and may have a gray, frothy appearance caused by small gas voids.

Mount Konocti obsidian
Associated with volcanic units south of Mount Konocti. Documented colors include gray, black, brown-black, and brick-reddish brown.

Erosion can move pieces away from their original flows and place obsidian pebbles in younger gravel, alluvium, creek deposits, and old lake sediments. That transported material is called float. Finding legal, naturally transported float is the safest goal for a recreational collector.

🔍 What Natural Obsidian Looks Like

Look for these characteristics together rather than relying on color alone.

Glassy luster

A fresh surface reflects light like dark glass. Weathered surfaces can become dull, cloudy, gray, brown, or lightly crusted.

Conchoidal fracture

Obsidian breaks in smooth, curved surfaces resembling overlapping shells. Ripple marks may radiate away from the impact point.

Razor-sharp edges

Fresh fractures can be considerably sharper than ordinary stone. Never sweep loose fragments with your bare hand.

Translucent thin edges

Hold a thin edge in front of a flashlight or bright sky. Black obsidian may glow smoky gray, brown, amber, or reddish along its thinnest edge.

Natural cortex

A naturally weathered piece may have a dull, pitted, brown, gray, or chalky outer rind covering a glassy interior.

Flow texture

Some pieces display faint bands, streaks, folded lines, tiny bubbles, or changes in translucency created while the lava was moving.

🧴 Obsidian or Bottle Glass?

Clear Lake recreation areas contain modern glass too, and yesterday’s soda bottle is rarely a Pleistocene eruption.

Obsidian is more likely when:

• It occurs with other volcanic rocks
• Several surfaces have a weathered natural rind
• The fractures are irregular and multidirectional
• Flow bands or natural inclusions are visible
• The piece appears embedded in undisturbed gravel or volcanic soil

Manufactured glass is more likely when:

• It is bright green, cobalt blue, or perfectly clear
• It has uniform thickness
• One face is perfectly flat or curved like a bottle
• Lettering, mold seams, bubbles, or machine-made ridges are present
• It occurs with cans, ceramics, or modern trash

Color by itself is not a reliable test.

⭐ Rare or Unusual Finds

These are possible regional varieties, not guaranteed finds.

Reddish-brown Konocti-type obsidian

Brick-red, brown-black, or reddish translucent material is less common than ordinary black volcanic glass and can be visually striking. Confirming the source requires geochemical testing rather than appearance alone.

Frothy gray Borax Lake-type obsidian

Some Borax Lake material contains small gas voids that give it a gray, bubbly, or slightly foamy appearance.

Flow-banded volcanic glass

Alternating glossy and cloudy layers may preserve movement within the cooling lava.

Obsidian breccia

Angular glass fragments can become enclosed within volcanic material during repeated breaking, movement, and cooling. The Clear Lake sources include massive flows, domes, and brecciated volcanic units.

Volcanic companions

The wider field can also contain:

• Rhyolite and rhyodacite
• Dacite
• Basalt and basaltic andesite
• Scoria and vesicular lava
• Pumiceous or ash-rich material
• Hydrothermally altered volcanic rock

These are part of the same volcanic orchestra, even when obsidian gets the guitar solo.

🧭 Where to Examine the Ground

Only use these methods after verifying that you are on land where collecting is permitted.

1. Naturally eroded gravel

Examine loose gravel that has already weathered from slopes. Dark glassy pieces often stand out after rain has washed away dust.

2. Dry drainage margins

Search loose material along the upper edges of washes rather than disturbing banks or digging into sediment.

3. Erosion below volcanic slopes

Obsidian is more likely to survive as angular float close to its source. Farther downslope, pieces become smaller, rounder, and more weathered.

4. Old alluvial gravel

Naturally transported obsidian may occur as rounded or subangular pebbles mixed with basalt, rhyolite, chert, sandstone, and other regional rock.

5. Previously disturbed legal surfaces

On confirmed BLM land, examine loose material beside existing routes or previously disturbed ground without widening roads, cutting vegetation, or entering closed areas.

Do not hammer cliffs, roadcuts, archaeological deposits, or bedrock exposures beside public highways.

🪶 The Artifact Rule

Clear Lake obsidian was used by Indigenous communities for thousands of years. Natural obsidian and worked obsidian can occur in the same general landscape.

Stop immediately when you see:

• Repeated flake scars
• A deliberately shaped edge
• A symmetrical point or blade
• A rounded striking platform
• Numerous flakes concentrated together
• Grinding stones, shell, bone, charcoal, or pottery
• Obsidian positioned beside other cultural material

Do not pick it up to “check whether it is real.” Photograph it where it lies, preserve the location, and report it to the relevant land manager. Moving an artifact even a few feet destroys archaeological context.

🎒 Suggested Gear

• Leather gloves resistant to sharp edges
• Safety glasses
• Sturdy boots
• Long pants
• Ten-power hand lens
• Small flashlight for testing translucency
• White index card for viewing edge color
• Hard plastic specimen container
• Newspaper or individual sample bags
• Permanent marker and locality labels
• First-aid kit with gauze and wound dressings
• Official BLM map plus offline GPS
• Plenty of water
• Fire extinguisher during dry season

Leave the rock hammer packed away until collection rights are completely confirmed. Loose float is enough for a productive scouting trip.

⚠️ Collection Rules

On ordinary BLM-managed California land, casual collection may be permitted when specimens are for personal, noncommercial use and the area is not closed, claimed, developed, privately owned, or otherwise restricted.

California BLM’s general limit is:

• Up to 25 pounds per day, plus one additional piece
• No more than 250 pounds per person per year
• Hand collection only
• No commercial collecting or bartering
• No unnecessary damage to the land
• No collecting from archaeological sites or active mining claims

Important for The Hidden Gem

Material collected under BLM casual-use rules is personal-use material. It should not be placed into shop inventory, sold, traded for business benefit, or incorporated into products for profit. Commercial sourcing requires separate authorization or material acquired from a lawful private source.

☠️ Mercury and Mine Safety

The Clear Lake region has a long history of mercury mining. Sulphur Bank Mine on the eastern shore is a federal Superfund site, and mercury contamination remains an environmental concern around the Oaks Arm of Clear Lake.

Avoid:

• Sulphur Bank Mine property
• Mine tailings and calcined ore
• Yellow, orange, or unusually powdery mine waste
• Warm springs and gas vents
• Abandoned structures
• Tunnels and adits
• Water draining from mine workings
• Dusty material around historic mercury sites

Do not cut, grind, tumble, or polish material suspected of coming from a mercury-mine area until its origin and safety are understood.

🔥 Seasonal Safety

As of July 2026, the BLM Ukiah Field Office has an active fire-prevention order due to increased wildfire danger. Conditions and restrictions can change rapidly. Check before every visit, even when you were there recently and everything looked peaceful. Dry California grass can transform from scenery into a fuse with alarming enthusiasm.

Also prepare for:

• High summer temperatures
• Limited cellular reception
• Rattlesnakes and ticks
• Sharp volcanic glass
• Rough, two-way dirt roads
• Hunting activity
• Sudden road closures
• Private-property boundaries