Which Rocks term is correctly associated with common use or association: “historically used for sharp cutting tools”, and also with “a volcanic glass”?

Correct Answer: Option D — Obsidian. Obsidian is correct because it is historically used for sharp cutting tools. Obsidian is also associated with a volcanic glass. A further identifying fact is that Obsidian is glassy texture with conchoidal fracture. Another useful fact is that Obsidian is associated with historically used for sharp cutting tools. Why the other options are incorrect: Option A — Sandstone: It is commonly used as a building stone and is associated with a clastic sedimentary rock; those are the identifying facts of Sandstone. Option B — Limestone: It is a major raw material for cement and is associated with a sedimentary carbonate rock; those are the identifying facts of Limestone. Option C — Shale: It is can metamorphose into slate and is associated with a fine-grained sedimentary rock; those are the identifying facts of Shale. Exam Takeaway: Link Obsidian with “historically used for sharp cutting tools” and “a volcanic glass” for quick revision.