Which mineral property can be determined simply by observation

Answer is: luster.

Luster is example of qualitative observations.

Qualitative observations are information about the quality of substances.  

In qualitative observation, informations are collected using senses (sight, hearing, taste, smell and touch).

Qualitative observations are not measurable and cannot be recorded in terms of numbers.

Luster is the way light interacts with the surface of a mineral.

Other example is cleavage, the tendency of mineral to split along definite crystallographic structural planes.

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56 moles of   lieter in chemistry