Learn more about rock classifications and compositions:

• Metamorphic Rocks
• Sedimentary Rocks
• Igneous Rocks: Intrusive & Extrusive

The website of Alex Strekeisen is a detailed resource on optical mineralogy, featuring hundreds of high-quality thin section photos of minerals and rocks viewed under polarized light. While it’s especially helpful for students studying mineralogy and petrography, collectors will also appreciate the wealth of information on compositions, associations, and formation environments. The site goes beyond identification—it includes valuable explanations of rock types and geological terminology making it a great tool for anyone looking to deepen their understanding of how minerals form and interact in different geological settings.

Mindat.org is a free, nonprofit resource and the largest publicly available mineral database. It includes detailed information on mineral species, photos, and localities, making it a valuable reference for collectors of all levels. The site is supported by contributions from the global mineral collecting and scientific community, and it's widely used for research, identification, and locality data.

• Ancient Earth
• Interpreting the Earth

Geological Digressions is a free, ad-free educational site featuring in-depth articles on sedimentary geology, structural geology, volcanology, and more. It offers detailed explanations, diagrams, and field photos that help readers understand rock-forming processes, geological structures, and terminology. While geared toward students and educators, anyone wanting to learn will find it useful for interpreting the geological context of specimens and gaining a broader understanding of Earth materials.

The U.S. Geological Survey (USGS) is a scientific agency that conducts research on the natural world, including geology, water, ecosystems, and natural hazards. Their Geology FAQs page offers clear, well-researched answers to common questions about rocks, minerals, earthquakes, volcanoes, and more. It's a great starting point for anyone curious about Earth science—from casual learners to serious collectors—providing reliable information straight from leading experts.