FMF - Friends of Minerals Forum, discussion and message board

[BenGrimm]

My grandfather just passed a month ago any left behind his collection. This rock is the largest and most mysterious so far. Any ideas what this could be? Very heavy, hard and dense

[James Catmur]

At the moment it is just a rock,

You need to take a photo of the cavity (assuming it has crystals in it) and do a few tests on the crystals to help us help you.

https://www.mineral-forum.com/message-board/viewtopic.php?p=19487#19487

James

[BenGrimm]

Is this close enough to get detail? What tests should be done?
Thank you

[Susan Robinson]

It looks like quartz, variety agate, to me.

[James Catmur]

I agree, agate.

No need for tests as it is solid, but you should find glass does not scratch it.

[Pete Modreski]

Yes, you clearly have a large rock nodule, with agate in the center. Being as it's evidently a large heavy rock, it's most likely your grandpa got it from somewhere in your local area, or at least, your state. Your profile doesn't say where you live, so, advice: when asking about what a rock or mineral is, it always helps if the give any info at all that you have, even if fragmentary or uncertain, about where it came from--in this case, all you have to go on, the state where you live.
And, a suggestion, worth a try: do a Google search for images of "agate" + your state, and see what you find, and perhaps you'll see some pictures of agate specimens quite like yours, and perhaps some ideas about what region within the state your rock came from.

[BenGrimm]

Thank you very much for the responses, that helps narrow things down. Grandpa retired in northern California coastal area, Humboldt county. He lived in southern California before that, so may have brought it from that region, or received it as a gift from another local rock hound here, hard to say for sure

[James Catmur]

Humboldt County does have agate localities, so it could be local. But unless someone who knows the area can help you, you may never find out the locality it is from