A rock is a collection of minerals and/or smaller rocks that are naturally coherent (attached together to form a solid).

Occasionally, a rock may be a solid formed by glass rather than mineral matter.

Observe & Discuss:

Station 1 - Which of these items is a rock?

  • Orange - Not a rock. Not entirely solid or made up of minerals and/or rocks.

  • Sea glass - Not naturally formed, or made of minerals or smaller rocks. While sea glass may have been smoothed by the natural action of seawater, the glass itself is manmade and thus not naturally formed./li>
  • Diorite - An igneous rock. Each speckle is an individual crystal of a mineral.

  • Conglomerate - A sedimentary rock. The particles in this rock did not form out of molten rock. They were solid pieces of rock and minerals before this rock formed.

  • Speckled plastic ball - Speckled, but not made up of minerals or smaller rocks. Not naturally formed.

  • Concrete - Solid, but not naturally formed. Formed from both preexisting solids and liquid, but not solidified from a molten state.

  • Loose sediment - Not a rock because these pieces do not form a solid altogether. However, the individual pieces are composed of minerals and/or rock.

  • Obsidian - Obsidian is an igneous rock composed of material that is a glass. Glass is formed by such quick solidification of molten material that mineral crystals do not have the time to form. Thus obsidian is not composed of mineral matter although it is a rock.