Final answer:
Alexandrite exhibits a color change from green to red, whereas tanzanite displays pleochroism with blue to violet hues. Other gems like ruby and emerald are also colored by chromium impurities, but their colors come from different base minerals. To accurately distinguish these minerals, properties beyond color, including hardness and crystal structure, should be evaluated.
Step-by-step explanation:
To use the diagnostic properties of alexandrite and tanzanite to distinguish them from each other and from other minerals, one must examine several physical properties. Alexandrite is a gem variety of chrysoberyl known for its unique color change; it appears green in daylight and red to purplish-red in incandescent light due to rare chromium ions. In contrast, tanzanite is a blue to violet variety of zoisite and exhibits distinct pleochroism, showing different colors when viewed from different angles. Unlike these gems, amethyst, citrine, and rose quartz are varieties of quartz and do not exhibit the distinctive color change or pleochroism of alexandrite or tanzanite.
Ruby and emerald are both colored by chromium impurities (Cr³+) within their crystal structures but differ in the base mineral; ruby is a red form of corundum (Al₂O3) and emerald is a green variety of beryl (Be₃Al₂Si₆O₁₈). When attempting to distinguish these gems, note that ruby's red color is due to its allochromatic nature, where trace impurities cause the color change. On the other hand, the color of an idiochromatic mineral, like peridot, arises from the essential components of the mineral. In summary while color is an observable property of minerals, it is not always diagnostic due to variations and impurities. Additional diagnostic properties such as hardness, specific gravity, refractive index and crystal structure should also be considered when identifying minerals.