Final answer:
A single coherent mineral with geometric faces is described as a crystal, with quartz being a quintessential example due to its six-sided geometric shape. Polymorphs share the same chemical composition but differ in crystal structure, altering their properties.
Step-by-step explanation:
A single coherent mineral with geometric faces is typically described as a crystal. A good example of this would be quartz, a mineral consisting of SiO2, or silicon dioxide. Quartz's crystal structure is what makes it distinct and recognizable. It has a non-metallic luster and forms hexagonal (six-sided) crystals, often large and well-shaped enough to present geometric faces.
A notable concept related to minerals is the idea of a polymorph. Polymorphs are minerals that have the exact same chemical composition but differ in their internal atomic arrangement and crystal structure. This variability in arrangement can result in different physical properties, even though the chemical composition remains unchanged.