It's tough to say definitively which physical property is *the most* useful for classifying rocks because it depends on the context and the level of detail you're aiming for. However, mineral composition is generally considered the most fundamental property used for rock classification.

Here's why:

* Defines rock type: The minerals present in a rock are the primary factor determining its classification (e.g., igneous, sedimentary, metamorphic).

* Provides insight into formation: The mineral composition reveals information about how the rock formed (e.g., cooling magma, weathering of existing rocks, heat and pressure).

* Relates to other properties: Mineral composition often dictates other properties like color, texture, and hardness.

That said, other properties are also important for identification:

* Texture: How the minerals are arranged (e.g., coarse-grained, fine-grained, foliated).

* Color: Can be a helpful initial indicator, but be careful, as color can be misleading.

* Hardness: The resistance to scratching (Mohs hardness scale).

* Cleavage: The way the mineral breaks along specific planes.

* Fracture: How the mineral breaks when not along cleavage planes.

Ultimately, the most useful property depends on what you're trying to achieve. For a broad classification, mineral composition is key. For more specific identification, you'll need to consider multiple properties.