Here is an excellent article decribing Biconoids by Chris Driesbach
The Biconoid is the symbol of the Santa Lucia Rockhounds and a fascinating rock. So what is it? The biconoid is a bi-cone shaped (imagine two cones put together) agate that is found in the fields of Templeton, California. It has striations that radiate from both sides of the bicone. These striations lead many to believe the biconoids are fossil bivalves, but rhyolite around some of the biconoids hints at their true identity as thunderegg cores.
The thundereggs formed as high silica magma was rapidly extruded to the surface leaving only enough time for one mineral to form, cristobalite. The cristobalite grows in a radial structure from a central point. As the lava cools the radial structure of the cristobalite shrinks and cracks through the center of the radial mineral structure. This leaves the void in a biconal shape that later gets filled with agate and quartz crystals. A radial pattern is left on the edge of the void because of the radial growth of cristobalite. This will become the striations on a biconoid when the cavity is filled. The agate and quartz are deposited in the void when hydrothermal solutions carrying silica seep through the rock.
As the formation was eroded the thundereggs with their biconiod cores were carried by river into the Templeton area. They are found in old river gravel deposits, as indicated by the water-worn cobbles that litter the collecting areas. There are numerous other theories of the biconoid origins that have little evidence to support them.
The biconoids and thundereggs found in Templeton come in a range of colors: pink, white, and red cores with tan, grey, blue or even purple matrix (rhyolite). Some of the best ones have saginite and agate tubes in the cores. They were once easy to find when they were uncovered by plowing and construction in the Templeton area, but now with many of the fields paved and built upon, they have become much harder to find.