GMAT Reading Comprehension (RC) | Diamonds are almost impossible to detect directly because

PrakharGMAT ,smartguy595 ,RDhin - may I offer my understanding on these issues?

Q1- The primary purpose of the passage is to
Before going to answer the question, my quick summary to the passage:

Diamonds are rare & hard to detect --> only through "kimberlite pipes" can have diamond --> prev: several ineffective ways to decide which kimberlite pipes have good potential of containing diamond --> later: Gurney's method is more effective.
So now to the answer:

(A)discuss
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