Final answer:
A rock sample is a mixture, specifically a heterogeneous mixture, if it has visible different components or varying composition throughout, as opposed to a chemically uniform pure substance.
Step-by-step explanation:
An indication that a rock sample is a mixture rather than a pure substance could be the presence of visible grains of different types of minerals or the lack of uniformity in its composition throughout. When different components in a rock sample are not chemically bonded and show varied compositions, it is considered a heterogeneous mixture. For example, if a rock contains different colored patches or layers with varied textures and materials, it is a clear sign of heterogeneity. Conversely, a rock consisting entirely of one mineral, with a consistent composition throughout, would be a pure substance.
Examples akin to rocks that would be heterogeneous mixtures include concrete, which is comprised of cement, sand, gravel, and water, or granite, which contains distinct crystals of quartz, feldspar, and mica. Each of these examples includes a mixture of substances that remain physically distinct and do not have a uniform composition throughout the material.