How to differentiate between a rock and a mineral by merely looking at it as a thumb rule?
If you see more than one mineral you frequently can call it rock, but not always. You could have a gem incrusted into another mineral or rock.
You need to know each rock uses to have one or more mineral -by definition-, but there are monomineral rocks as limestones, wich can be only composed by calcite. If you give me a limestone I could say I have a rock, and I see also calcite, normaly with a magnifying glass.
When we are searching for minerals or thinking on minerals, we use to search for well formed species. If you see a big cristal, you can affirm it is a mineral. Sometimes they are twinned. If you see an aggregate of little minerals, most times you have a rock independently if it has one or more mineral types but not all times again.
What characterizes a rock at surface uses to be diagenesis -compaction of minerals- if the rock is sedimentary. Igneous and metamorphic rocks are also well formed mineral aggregates, consistent bodies.
Sedimentary rocks have a lot of minerals/mineraloids cemented by a kind of microcrystaline matrix -that are also minerals crystallographically talking!-.
Igneous and metamorphic rocks are compact bodies were there is a texture, minerals have textural relation (some kind of order).
If you are at an ore, you could be confused again if you don't know basis concepts. You then could think the aggregates of minerals are rocks, wich is not the case. Then you have an aggregate of well formed minerals, wich is not the same as a rock.
There are several definitions of rock and mineral. I took this ones:
Rock:"mineral matter of variable composition, consolidated or unconsolidated, assembled in masses or considerable quantities in nature, as by the action of heat or water."
Mineral:"any of a class of substances occurring in nature, usually comprising inorganic substances, as quartz or feldspar, of definite chemical composition and usually of definite crystal structure, but sometimes also including rocks formed by these substances as well as certain natural products of organic origin, as asphalt or coal."