This hard, silicate sedimentary rock is similar to flint but with a less ordered structure. It is a cryptocrystalline form of quartz and is known to be extremely hard on drill bits.

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

Answer: (B)

Chert is the rock being described. It’s a hard, silicate sedimentary rock made of microcrystalline quartz, so the crystals are so small they’re not visible to the eye—this cryptocrystalline texture gives it a very fine, dense grain. Because the quartz content is high and the crystals are tightly packed, chert resists weathering and can take a sharp edge, which is why it’s noted as being extremely hard on drill bits. Flint is a specific variety of chert found in chalk, but the clue about a less ordered structure points to the broader chert category rather than the more uniform look often associated with flint. Quartzite, by contrast, is metamorphic, not sedimentary, and chalk is a carbonate rock, so they don’t fit the description.