Here's why:

* Minerals have a defined chemical composition: This is the fundamental property that defines a mineral.

* Thickness is variable: The thickness of a mineral sample can change based on the way it was formed, how it was broken, or how it was processed. It's not a consistent property of the mineral itself.

Instead of thickness, we use these characteristics to identify minerals:

* Color: While color can be misleading, it's a starting point for identification.

* Streak: The color of the mineral's powder.

* Hardness: Resistance to scratching.

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

* Fracture: How a mineral breaks irregularly.

* Luster: How light reflects off the mineral surface.

* Crystal shape: The geometric form a mineral crystallizes into.

* Specific gravity: The mineral's density relative to water.

Remember, identifying minerals requires a combination of these characteristics, not just one or two.