Found several different minerals (rocks?) in the lower peninsula of Michigan, near Bay City. It was found near train tracks, so could have come from any quarry in the lower peninsula.
This first piece is soft enough to be scratched by a steel knife. I have not checked its reaction to acid, but it appears to be limestone. The white material is the mystery. Maybe some kind of trace fossil?
This second piece is what I can only guess is Pyrite on Limestone (or something very similar). Most of the pyrite looks good, but some looks very "rusty," so I am unsure if these are pyrite that really needs to be cleaned, or just iron.
close up
a rusty type looking chunk
IF these rusty parts are Pyrite, how can I clean them up? I was going to put them in Iron Out, then quickly realized that Pyrite itself could also disappear. the first mineral actually appears to NOT be limestone! Slightly difficult to scratch and no reaction to acid. 
I am not sure if the pyrite is interesting enough to clean. It appears to be massive or broken. Perhaps iron out can clean it up a bit if you really want.
Your first piece could be dolomite and might dissolve if crushed or with warm acid otherwise mindat says it dissolves very slowly. It could also be silicified and that could cause it not to dissolve. Yeah, I wasn't sure if the Pyrite was worth cleaning. Some of them are all cleaned up when I find them, so we'll see.
The first piece DOES bubble when crushed into a finer powder, so that's good to know. So calcium carbonate, dolomite, limestone, etc? 
Bob,
your first specimen is a piece of limestone from the Wallace Stone Quarry in Bayport. The white markings in it are filled in burrows- a trace fossil. The material is common in parts of the quarry. I agree that the rusty stuff is pyrite, and agree with Matt that it's probably not worth spending any time on. 
Agree with Chris and Matt..... If the carbonate fizzes after it is powdered, then likely dolomite. 
Robert;
In Manitoba, Canada we have a similar rock, Tyndall Limestone, which is used extensively as construction and facing stone including our nation's legislature buildings. The only difference is the rock is mostly calcite (limestone) and the colour contrast is reversed with the burrows being dark and the matrix being of lighter colour.
Also when acid testing carbonates it is necessary to be consistent. The relative reaction rates between the more common ones, calcite, dolomite and siderite, is diagnostic. It is important to familiarize yourself with the relative reaction rates by using the same type and strength of acid and testing on know minerals. I might suggest 10% HCl and compare the relative reactions of calcite (limestone) and dolomite (or dolostone). The Wallace Stone Quarry limestone should have had a very vigorous reaction to acid if it is mostly limestone (calcite). If the reaction is slow to start and then builds with time, it is likely dolomite or dolostone.
