* Coal: Coal is a sedimentary rock formed from the accumulation and compaction of plant matter over millions of years. It's primarily composed of carbon, along with other organic compounds, and doesn't fit the strict definition of a mineral (it doesn't have a specific, defined chemical composition).
* Obsidian: This volcanic glass is formed when lava cools so rapidly that crystals don't have time to form. It's not technically a mineral because it lacks a crystal structure.
* Amber: This fossilized tree resin is often found containing insects or other organisms. It's primarily composed of organic compounds and doesn't have the defined structure of a mineral.
* Tektites: These glassy rocks are formed when meteorite impacts melt and vaporize terrestrial rocks. They are essentially glass shards, lacking a crystalline structure.
It's important to note that even rocks like obsidian and tektites, though not technically minerals, are still considered rocks because they are naturally occurring solid materials.
