FMF - Friends of Minerals Forum, discussion and message board

[Aetherian]

I found this buried pretty tightly shining at bottom of river here in Washington State

[James Catmur]

It is a large rock containing quartz

[Pete Richards]

I think this might be a pyrite nodule, composed largely of pyrite (fool's gold), iron sulfide, which is quite dense as mineral go.

So I'm guessing this is rather heavy for a rock of its size. If you had some way to estimate its volume, then we could calculate an average specific gravity for the entire stone.

One way to do this: Fill a bucket to the top with water. Immerse the rock in the water. An amount of water equal to its volume will spill out (do this outside...) Remove the rock. Using a measuring cup, fill the bucket to the top again, keeping track of how much water it takes to fill it. That's the volume of the rock. The specific gravity is the weight divided by the volume. Any value greater than about 3.0 would be above average for typical rocks and minerals. Pure pyrite would be about 5.

[Pete Modreski]

I'll agree, that this is most likely pyrite, Aetherian. Likely mixed with quartz (if there's obvious white mineral material with it). If the whole rock is pyrite, it should feel REALLY heavy.

If it were gold (VERY unlikely), you'd be able to scratch the golden-metallic material easily, with the point of a knife or a nail. Pyrite is hard enough, it won't scratch.

Another test, if you break off a crumb of it, hit is with a hammer on a hard surface and pyrite will shatter to a powder, which will look more of a greenish-metallic color. And you likely will smell a whiff of sulfur, if you powder it that way.