1. Minerals: The Building Blocks of Rocks

Before we can understand rocks, we need to meet their ingredients: minerals. People often use 'rock' and 'mineral' to mean the same thing, but to a geologist they are different.

A mineral is a naturally occurring, inorganic (non-living) solid with a definite chemical makeup and an orderly crystal structure — for example, quartz, calcite, or feldspar. A rock is a solid made of one or more minerals packed together. So a rock is like a fruitcake, and the minerals are the nuts and fruit inside it.

Because each mineral has its own properties, geologists identify minerals by testing them:

• Hardness — resistance to scratching, ranked 1–10 on the Mohs scale (talc = 1, diamond = 10). A mineral scratches anything softer than itself.
• Luster — how the surface reflects light (metallic vs. glassy/dull).
• Streak — the color of the mineral's powder when rubbed on a tile (often more reliable than the mineral's color).
• Cleavage — whether it breaks along smooth, flat planes or into uneven chunks.

⚠️ Common misconception: color is not a reliable way to identify a mineral. The same mineral (like quartz) comes in many colors, and different minerals can share a color — that's why streak and hardness are more trustworthy.

💡 Tip: a mineral is one pure ingredient with a fixed recipe; a rock is a mixture of mineral grains. Mohs scale: the higher the number, the harder.