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Description: This rock contains a feature called a hydrothermal vein. This is the result of hot fluids (mostly water, with lesser amounts of other fluids like CO₂, H₂S, and N₂) fluxing through the crust. When the hydraulic pressure is sufficiently high, rocks can fracture allowing these hot fluids to pass through the newly formed fracture networks. These hot fluids contain abundant dissolved materials which precipitate to form minerals in these fractures as the fluid is depressurized and cooled. The most common vein-forming mineral is quartz (SiO₂). During rock fracturing, pieces of the surrounding rock can be broken off and integrate themselves into the vein, as can be seen in this sample. These rock fragments now locked into the solid veins are called ‘inclusions’ or ‘xenoliths’.