Final answer:
The mineral property that is easily observed but unreliable for identification is color due to variations caused by impurities.
Step-by-step explanation:
The mineral property that is easily observed but can be unreliable when it comes to mineral identification is color. Color is often affected by small impurities in a mineral's chemical composition, which can lead to a wide range of colors for the same mineral. This makes color a diagnostic challenge in mineral identification.
For example, quartz can exhibit almost any color depending on such impurities. While color is the most straightforward to observe, it isn't a reliable indicator because the same mineral can appear in a variety of colors due to these impurities. As an alternative, properties like streak, hardness, and cleavage are also used to identify minerals. Streak, for instance, refers to the color of a mineral in its powdered form and is more consistent than the color of the mineral itself.