Two pieces of sedimentary lace rock. Unused for years (details below). Could be a great piece of hardscape for your next build! It will likely still be heavily mineralized, so I would recommend against using it in a tank where soft water is desired.

Disclaimer: 

This used to be part of a marine tank it could still have aragonite pieces, salt crystals, and/or old silicone imbedded in it. I will clear off as much as I can, but I make no promises it will be 100% pristine.

Where it's from:

This piece of lace rock comes from the hardscape of a built-in marine tank that was part of the house my father-in-law used to own. To my knowledge, it hasn't been part of a marine ecosystem for 15+ years. Two years ago, when my father-in-law was selling the house he asked me to tear down the scape. When I got home I soaked it in tap water for a few days, then vigorously hosed it off 2 or 3 times. It has been dry and awaiting use since then.