This gemstone reference chart covers the main mineral families, physical properties, and identification tests used in Earth science. Students need this cheat sheet because many gemstones look similar but have different compositions, hardness values, and crystal structures. It helps connect attractive gem materials to the mineral properties scientists use for classification.
The chart is useful for lab work, rock and mineral units, and quick review before assessments.
The most important concepts include Mohs hardness, streak, luster, cleavage, fracture, density, and crystal form. Gemstones are identified by combining several observations rather than relying on color alone. Common families include quartz, corundum, beryl, feldspar, garnet, and carbonate minerals.
A careful identification process compares visible traits with simple test results to narrow down the possible mineral.
Key Facts
- Mohs hardness ranks scratch resistance from 1 to 10, with talc at 1, quartz at 7, corundum at 9, and diamond at 10.
- A mineral can scratch any mineral with a lower Mohs hardness, but it cannot scratch a mineral with a higher Mohs hardness.
- Streak is the color of a mineral’s powder and is often more reliable than surface color for identification.
- Luster describes how a mineral reflects light, such as metallic, glassy, pearly, silky, or dull.
- Cleavage is the tendency of a mineral to break along flat planes, while fracture is breakage along uneven or curved surfaces.
- Specific gravity compares a mineral’s density to water and can be estimated by specific gravity = mineral density / water density.
- Gem color can change because of trace elements, such as chromium making ruby red and iron or titanium helping make sapphire blue.
- A gemstone is usually a mineral valued for beauty, durability, and rarity, but some gems such as amber and pearl are not minerals.
Vocabulary
- Gemstone
- A gemstone is a natural material, usually a mineral, that is cut or polished because of its beauty, durability, and value.
- Mineral
- A mineral is a naturally occurring, inorganic solid with a definite chemical composition and an orderly crystal structure.
- Mohs Hardness
- Mohs hardness is a 1 to 10 scale that ranks how well a mineral resists being scratched.
- Luster
- Luster is the way a mineral’s surface reflects light, such as glassy, metallic, pearly, or dull.
- Cleavage
- Cleavage is the tendency of a mineral to split along flat, repeating planes of weakness in its crystal structure.
- Streak
- Streak is the color of a mineral in powdered form when rubbed on an unglazed streak plate.
Common Mistakes to Avoid
- Identifying a gemstone by color alone is wrong because many different minerals can share the same color and one mineral can occur in many colors.
- Confusing cleavage with fracture is wrong because cleavage creates flat repeated surfaces, while fracture creates uneven, curved, or splintery breaks.
- Assuming a harder gemstone is always more valuable is wrong because value also depends on rarity, clarity, color, size, and demand.
- Using streak on very hard gemstones without caution is wrong because minerals harder than the streak plate may not leave a useful powder and can damage the plate.
- Calling every shiny colored rock a gem is wrong because a gemstone must have suitable beauty, durability, and rarity, and many colorful rocks are too soft or common.
Practice Questions
- 1 A mineral scratches fluorite with hardness 4 but does not scratch apatite with hardness 5. What is the mineral’s approximate Mohs hardness range?
- 2 Quartz has a hardness of 7 and topaz has a hardness of 8. Which mineral can scratch the other, and why?
- 3 A sample has a mass of 52 g and a volume of 20 cm3. What is its density in g/cm3, and how could that help identify the gemstone?
- 4 Two gemstones are both blue and glassy, but one has hardness 7 and no cleavage while the other has hardness 9 and no cleavage. Explain why color and luster are not enough to identify them.