There is quite a bit of variation in where the beach rocks are found and which ones are where. Certainly the beach is big time rocky with rhyolite near Charlies Rock and down the coast for a distance. Good rhyolite cobble can be found all the way toward Pilar and Bahia Delfin, but its most abundant on the high tide mark closest to Charlie's Rock. About half way to Pilar from downtown San Carlos, is a hot spot for some rather unusual white shell like structures. I have work the beaches on the back side of Tetakawi, but the green and white rocks predominant there were disappointing when slabbed. Towards La Manga the beaches are predominantly sandy, but I'm sure there are sections where rocks accumulate and await discovery. Among the rhyolites there are so many vaiations, but the ones with the maximal contrast are my favorites.
For example.
And after being tumbled:
I've never determined what the source of these white shell like medallions is, but the may be extremely worn remnants of large shells. They are slightly concave/convex, have no trace of a hinge as would a bivalve, have a radiating growth form as evidenced by a distinct crystalline structure and every one is different. Not an operculum or a radula in my eyes.
Here is one that I have accentuated the detail on with a bit of leather dye.
