bottle rock road obsidian
What you can find: Black obsidian; more rarely, mahogany and snowflake obsidian.
Ease of access: Very easy to access, the material is right in the road banks.
Equipment needed: Gloves. Nothing else needed if you're content to pick up loose pieces. If you want to break rocks, take eye protection and a rock pick.
Safety considerations: Obsidian is glass, and shards will cut you easily. Wear gloves, and if you're breaking rocks, eye protection too.
Where, exactly? Near Kelseyville, on Bottle Rock Road, around 2.7 miles from the junction with Highway 29/175.
Obsidian is a form of volcanic glass, created when felsic lava from a volcano cools too quickly to form any significant crystal structure.
Because it is a glass, albeit a naturally-forming one, it breaks like any other glass does, into thin, sharp shards. Care must be taken when picking pieces up, for this reason - it's best to wear gloves, to avoid painful cuts.
The name of the road supposedly came from all the rocks that looked like broken glass bottles, hence "Bottle Rock".
The Lake County obsidian was very popular with the Native Americans for making arrowheads and other sharp tools, and they traded the material far and wide - obsidian arrowheads have been found all the way over on the Mendocino Coast, many miles from the nearest formation. Even today, many hobbyists like to use obsidian for knapping arrowheads.
From the junction of Highway 29/175 with Bottle Rock Road, travel south around 2.7 miles, and look in the roadsides. You'll see an abundance of shiny black rock poking out of the red soil - find a safe place to pull off the road, and walk along the roadsides until you find something that you like enough to take home!
Black is the most common color to be found here, but I've found chips of mahogany obsidian [red-orange, usually with black splotches], snowflake obsidian [black with white patches], and some that has a very pretty silvery sheen to it.
You'll certainly find something to meet your fancy on Bottle Rock Road. And as long as you're in Lake County, why not try your luck with some Lake County Diamonds as well?
Good luck and happy hunting!