[johnproctor (OP)]

After collecting hundreds of California beach agates my brother and I became curious.  Sometimes the rind is thick (1mm), sometimes thin, sometimes absent.  In the thin cases, the rind picturesquely follows the patterns of the agate lines, but with two distinct cases:  Sometimes blurred as if eroded, and other times very delicate and precise like frost on a window.  So we imagined the agates first in a quiet state (e.g. submerged in a sandbar) where the rind builds up gradually in a dissolving process, either thick or thin, then once they are exposed it disappears pretty quickly via the pounding erosion of the surf.  Is this the actual process or are we way off?  And what's the time scale?  We're guessing millennia for rind buildup and days/weeks for erosion, but we really don't know.   

[JimBob]

They are most likely pisolites - carbonate secretions around grains of g being rolled around in a h energy environment and then selling again on a lower energy environment, i.e., being inside the sand bar instead of on top.

That is a first guess. If you are serious about finding out, find some 10%  hydrochloric acid and see if the beach rind fizzes. Most good rock shops have 10% HCl. Or get a kid to get some from his high school chem lab.

NOW - I could be completely wrong as I do not have a clue what a California beach agate is.  

I don't know when I will be able to check this again as I am in the hospital trying to get a bit better. But I will check in.