So my friend hands me this hunk of something hard and says it's a rock. Immediately, I recall what I had learned in Geo-science about how to determine what is a rock and what is a mineral. People assume that if it's smallish, round, and hard it's a rock. But actually it will always be partly a mineral, seeming as how all rocks are made up of minerals.
Now I had confused my friend. Yes, all rocks are minerals, but there are other minerals that are just minerals. So she asked me how she would know if it was just a mineral. Minerals are made of inorganic materials and usually have a distinct color, like yellow, white, clear, green, pink or black. They also contain a luster, which makes the mineral look metallic, glassy, waxy, or pearly. If you really want to make sure you can examine the streak, or the powdered form of the mineral. You could also look at the crystal structures, hardness, cleavage, and fracture.
My friend understood, but then wondered how you know if what you're looking at is a rock. I told her that rocks are made out of minerals and are categorized on how they form. This can be from cooling of molten rock (igneous), compaction and cementing of sediment (sedimentary), or other rocks changing due to heat and pressure (metamorphic). They can be recognized by their grain size, mineral composition, and texture.
I then asked my friend to re-examine and tell me if it was a rock or a mineral. The hunk that she was holding was a brassy gold color that looked very metallic. When scratched on the sidewalk it left a greenish streak. My friend told me that it was a mineral. I said "Correct! That's pyrite! It's also known as fool's gold! Congrats junior geo-scientist!"
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