If a channelway for hydrothermal solutions is left incompletely filled with mineral matter, there may be cavities, or “vugs” in the deposit which will be lined with inward pointing crystals. These are quartz crystals, showing mainly the pyramid (or, more exactly, rhombehedron) faces.
The quartz was deposited on a rounded surface of hydrothermal hematite (Fe2O3) and is coated with tiny crystals of the same mineral. – Specimen card
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Emily Zawacki, Ph.D. 