What does 'high grading' refer to in mining terms?

High grading in mining terms specifically refers to the practice of selectively extracting only the best or highest-quality portions of a mineral resource, such as gemstones, while leaving the lower-quality materials behind. This practice usually occurs when workers illegally take higher-grade materials for personal profit, often neglecting the ethical and legal implications of their actions.

While the other choices relate to different aspects of mining and gem processing, they do not encapsulate the concept of high grading as accurately. Extracting quality gemstones denotes a legitimate practice in mining, rather than the illicit act implied by high grading. Evaluating gem quality refers to the assessment process used to determine the value of gemstones, which is important but distinct from the concept of high grading. Sorting gemstones by size involves categorizing them based on their dimensions, an important part of the gem distribution process that does not address the selective and often dishonest nature of high grading.