- Questions below guide your observation of a sample for relevant properties.
Observe the rock sample for texture. Which texture does the rock most closely resemble?
- crystalline - Does the sample exhibit visible (phaneritic) crystals? A mix of visible and microscopic crystals (porphyritic)? Just microscopic crystals (aphanitic)?
- vesicular - Does the sample exhibit holes? More rock than holes? Or, more holes than rock?
- fragmental - Does the sample exhibit a mixture of broken fragments floating in a matrix of ash? Or just fine-grained ash?
- glassy - Does the sample exhibit conchoidal fracture?
Composition is most relevant for crystalline texture.
- Are individual crystals visible - phaneritic - or invisibile - aphanitic?
- Are scattered larger crystals floating in a fine-grained (aphanitic) groundmass (matrix)?
- What is the tone (lighter or darker) of the rock?
- Is the rock more felsic (lighter tone), mafic (darker tone), or intermediate?