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So I have a little dilemma. I was gifted with some lava rocks and they look fantastic, extremely porous and full of character. However, they are really, really light, not in colour but in weight. So light in fact that they float.

Yup, I have rocks that FLOAT!!!

I mean seriously, my rocks float. Has this happened to anyone else, if so what do you? Can they be sunk like driftwood? If not, I guess I'll either use them in the garden or I'll grind them down and use them as a plant substrate. Provided that they will sink when ground up.

Any assistance would be appreciated.
 

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Is it pumice? Also called feather rock. It's white and floats in water for weeks before sinking, large pieces can stay afloat for months.
 

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Yep, pumice.

pretty neat rock. Another nice aspect, is it's really easy to shape/scuplt (just don't breath the dust).

It's pretty much foam made of volcanic glass.

If you know how you want to position the rock, you might be able to glue/bolt it onto a piece of slate or ceramic tile.

One concern would be that it's often really abrasive. If your piece is well weathered, this may be less of an issue, but if you don't end up getting a bunch of moss/algae on it, there is some potential that it could cut/scrape fish if they happen to rub up against it.

I think if you grind it down small enough it will sink (I've got some waterlogged pebble-sized pumice I'm suing as filter media, and it stays submerged now.) The bigger pieces are more likely to almost permanently float, as the gas bubbles are completely sealed.

I've got a boulder I picked up at a landscape supply place and was hacking up, I'm trying to decide if I want to use that or the old epoxy-sand-styrofoam method for a paludarium I'm hoping to start on in the near future.

Might be able to make an interesting floating planter out of it...
 
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