Anyone know what this rock type is? Safe for tank?

[Bubba643]

So I don't want to jinx it but tomorrow I should be getting my new bow front 180 gallon (well Craigslist new). I'm pretty pumped (wife not so much)so I decided to go backyard decor scouting. I found these around an old concrete koi pond that I hadn't ever messed with in the 15 years I've owned the house. Just not sure what they are. They almost look man made, like a hardened glue. First impression they look like an aquarium bio media paradise.
Anyone know what they are? Light weight, I moved all but the big one palming them with one hand. They float which is odd but ones that were half buried sink. Hopefully pic shows close enough for you guys to see texture... if they are aquarium safe I'm going into full aquascape mode on this 180...

 

[TMaier]

I believe it is Feather Rock. Or an old school name is pumice. Natures own puffed glass.
If you are concerned about it harboring undesirable minerals you could try soaking it and testing the impact on the water.

 

[flchamp89]

I believe it is Feather Rock. Or an old school name is pumice. Natures own puffed glass.
If you are concerned about it harboring undesirable minerals you could try soaking it and testing the impact on the water.

Nailed it

 

[TMaier]

I have seen some amazing feather rock aquarium sculptures.

 

[flchamp89]

Ive used it as bio filter lol. But ya its great rock

 

[Bubba643]

Yep looks like it. Thanks for the info. There's about 15 of them so I'll probably use them for both in tank and 1 or two smaller bolders in sump as tank starters down the road. Awesome!