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Hydrothermal veins are a combination of the two ways that crystals form. Magma contains water as well as molten rock. Because it is underground, and under tremendous pressure, the water stays a liquid. At very high temperatures and pressure, that water can dissolve quite a few minerals. As the magma cools, the last part that is still a liquid is the quartz and the high temperature/high pressure water. They flow into cracks in the surrounding rocks, where they cool. The quartz starts to solidify quickly, but the hot water keeps some of it dissolved.
When we tested hardness, we were trying to scratch the specimen, not break it. For the next two tests, fracture and cleavage, we will be breaking the specimens. We are not looking at how difficult it is to break them. Instead, we will be looking at how they break, especially at the shape of those broken surfaces. Minerals that break in flat, smooth planes have a property called cleavage, and we will look at those in the next test. Any kind of break that is not cleavage is called fracture, and there are several different kinds.
