Rock sample dichotomous key

This dichotomous key separates four rock samples using observable texture and one simple test. A glassy surface points to obsidian. For the other rocks, a dilute-acid reaction distinguishes limestone. Visible sand-sized grains indicate sandstone, leaving granite as the crystalline, non-fizzing sample. The questions are arranged so each one narrows the remaining options. In a classroom, explain safety before using acid and substitute a different property if testing is not appropriate. Real rock identification can require additional observations, but a short key is useful for learning how classification decisions work. It gives readers a concise reference they can reuse when discussing the result with colleagues.