Based on what I can make from thinking and mental observation, I think the reason being that coal isn't considered a mineral is that it is carbon based. In other words, its made from organic organisms (decayed organisms) which means that it doesn't have its own set of a defined element. While a mineral such as Iron (FE) has an atomic number of 26, electric configuration and a set atomic weight of 55.845. If there is an element feature such as this, you know for sure its Iron. Note: I got the element details because I have a periodic table chart right in front of my computer