The field guides generally show only a handful of rocks, and their pictures show crystals at their 'best' with perfectly formed crystal structure, not what you normally encounter in the field. To complicate matters, many rocks and minerals as well, are known by their trade names, names given by local rockhounds and mineral dealers, not a name that is recognized as a mineralogical name in any real sense. Color is often used to identify rocks and minerals. Many minerals have a distinctive color, however, many minerals can occur in several colors. For example, calcite may be green, yellow, blue, and red.
Density refers to the weight for a given volume of mineral.
You can get a good feel for a mineral's density by comparing minerals that are a similar size, and seeing how much they weigh. The hematite is a very dense mineral- it has a high weight per volume. Fluorescence is a unique property of some rocks and minerals that can only be observed in ultraviolet light. The fluorescent minerals and rocks can absorb ultraviolet light, and re-emit that as visible light. Fracture a fracture is a break in a rock, or a rupture that has occurred in the rock.