Which rock type is a cryptocrystalline silica and known for its hardness on drill bits?

Which rock type is a cryptocrystalline silica and known for its hardness on drill bits?

• A. Quartzite
• B. Chalk
• C. Flint
• D. Chert

Answer: (D)

Chert is a cryptocrystalline silica rock, meaning its quartz is made of tiny crystals you can’t see with the naked eye. That microstructure gives it a very high hardness—about 7 on the Mohs scale—and a conchoidal fracture, so it resists scratching and can take a sharp edge. These traits make chert well known for its durability in cutting and drilling contexts. Flint is a specific variety of chert found in chalk, so the general rock type that fits the description is chert. Chalk is calcium carbonate, and quartzite is crystalline quartz with visible grains, not cryptocrystalline silica.

Chert is a cryptocrystalline silica rock, meaning its quartz is made of tiny crystals you can’t see with the naked eye. That microstructure gives it a very high hardness—about 7 on the Mohs scale—and a conchoidal fracture, so it resists scratching and can take a sharp edge. These traits make chert well known for its durability in cutting and drilling contexts. Flint is a specific variety of chert found in chalk, so the general rock type that fits the description is chert. Chalk is calcium carbonate, and quartzite is crystalline quartz with visible grains, not cryptocrystalline silica.