Beach agates, sometimes also called "moonstones" [although this name is incorrect - true moonstone is a form of feldspar] are a common find on California's coasts. Though they are much sought-after, many beginning rockhounds are left wondering just what an agate looks like, and if they've actually found any.
I often get emails from readers asking "Is this an agate? How about this?" And although I'm happy to answer such questions to the best of my ability, I thought perhaps a blog entry was warranted.
The difficulty with agate is that it comes in many colors, from clear to yellow to orange/red or even blue. The first clue I look for is the white coating that many beach agates have on the outside, which is often etched into intricate patterns by the elements [see photo above for an example].
Next hold it up to the light, and see how the light shines through it. Agates seem to almost glow, more so than any other stone I know.
They will feel very hard [around 7 on the Mohs scale] and smooth-grained. Look for bands within the agate [not all will have it, and sometimes it's faint, but a lot of them do have it]
The most common rocks I see people mistake for agates are quartz pebbles, banded rhyolites, and chert. Remember, if it's not translucent, it MIGHT be agate but it's not GOOD agate, since the real beauty in the stone is how it transmits the light.