That is an iris rhizome.

Here is a link that might be useful: Iris

Yes, iris

Hi,

Iris is pretty tough. I have quite a few that even do well in a bed of dry soil. They root pretty close to the surface and so when they start multiplying they should be divided. With a big clump of iris, you'll often see these rhizomes right at the ground level. With any luck, you'll have flowers by next spring!

Keith

When you plant it, please put in the soil the other way round, so that the green side is up. ;)