Rocks and specimens from California

Curb Showing Hayward Fault is Replaced

Hayward, California is the home of the Hayward Fault, which is a break in the Earth’s crust. Geologically speaking the Hayward Fault moves very fast. You can see that the tectonic plates are always moving when you look at the roads. Since it only moves a few millimeters a year, the roads and/or curbs don’t crack but instead gently wiggle apart at the seams.[*] See how this curb at Rose and Prospect has moved over time?

curb a few inches offset hayward fault

This is how the curb looked in 1974. It has already drifted several inches from where it was originally. Photo from

hayward fault curb 2012 with people standing on top

This is how the curb looked in May 2012. See how far this piece has moved away? Photo by Andrew Alden at

If you want to go to California and see it for yourself, you are out of luck. The city has just replaced the curb. This is probably the first time that road work has made it into the New York Times. Read the article here: A Curb is Repaired and a Seismic Marker is Lost

[*]Note: Cracks in the road are not caused by plate tectonics. They are caused by thermal expansion/contraction, road salt, heavy trucks, and degredation of the road bed.


Corundum (Al2O3) is a hematite group mineral that has trigonal crystals. It is found all over the world and can be many different colors including blue, red, pink, yellow, gray, and colorless. These corundum crystals are from the Cascade Canyon in San Bernadino, California. They may not look familiar to you, but corundum has some famous relatives. A gem-quality corundum that is red (Cr-bearing) is known as ruby, and a gem-quality corundum that is blue (Fe- and Ti-bearing) is known as sapphire.

Agates and Jasper

Jasper and agates are both a type of quartz known as chalcedony. So it makes perfect sense that sometimes you can find jasper and agate melded together, like so. Agates are usually transparent or translucent, and jasper is usually opaque, but this isn’t a hard and fast rule. Sometimes it’s very difficult to tell the difference between agate and jasper, in which case people may call it chalcedony, agate jasper, or jaspagate. It is mostly found in the American Southwest and California, like other agates and chalcedonies. The one pictured was found near the San Andreas Fault.

Sagenitic Agate

Sagenitic agate is any agate having acicular or needle-like mineral growths inside.  These hair-like filaments are often arranged in fans or starbursts.  The inclusions come in a wide array of colors.  Sagenite has been found in over 250 different agate deposits worldwide, a little in most agate fields, probably less that 5% of the available agate in most fields.  This agate is from California and contains mostly yellow sagenite but also a few plumes; can you spot them?

Source: The Gemrock, 7/2014

Update: It looks like somebody at IGAMS (and the CMS Tumbler, The Clackamette Gem, and The Glacial Drifter, all of whom were listed as sources) might be a fan of Pat McMahan’s website Agates With Inclusions, as the text from their article on agates that I read is very similar to this article on sagenite and plume on his website.  So, thank you for the inspiration, Mr. McMahan!