Rock found along northern Lake Michigan, (Charlevoix, MI). Made up of very thin crystalline layers. There are small, round bubble like bumps that protrude from the surface. Doesn't show well in the picture, but the rock has a sugary appearance.
I can't be definite but my three best guesses are Travertine, Agate and maybe Halite, if it fizzes in mild acid it's Travertine, a form of Limestone, if it dissolves in hot water it's Halite, or Rocksalt, otherwise if it's more or less inert it's probably Agate, an amorphous silicate. I find Halite unlikely, the other two are probably pretty equally likely in that location.
This rock looks very similar to a Petoskey stone which is the state rock of Michigan. The rock is made of the six sided coral Hexagonaria percarinata. They were deposited in this area during the Pleistocene glaciation around 2 million years ago. This fossil is distinctive of the Travis group strata in the Petoskey and Charlevoix regions of Michigan. They come in many sizes and the hexagonal patterns really pop when wet or polished. More information can be found at the wikipedia link below.
Stromatolite structure. The graininess a beat up or weathered limestone of some sort. But if it scratches glass it may be dolomatized limestone with some silica grains, enough to scratch glass. Stromatolite is often of chert but I haven't seen chert weathered like this. The layering looks like chert too. It's a puzzle.
Stromatolite structure. The graininess a beat up or weathered limestone of some sort.