This Wikipedia page will give you a good start on concretions.
Also, I just noticed the part where you said it rattled. That may make a type of concretion called a septarian nodule.
I personally never use iron discrimination. The discrimination works by measuring both ferrous properties and conductive properties of a given target. Since many gold nuggets are coated with ferrous material when you dig them up they can be missed when discriminating. That's why most of the higher end gold detectors don't even offer that feature. Also, the discrimination does not work well for strongly ferrous targets (if I recall correctly from my old GB2 days...)
So a question for the crowd. I was working the bed rock in a dry creek. There a few spots with water and when I swung over them you get a signal. Is that normal or can you turn the water out? Not sure if I explained that good enough.
May rain so may get so more time to hunt.