A more thorough explanation:

Definition: To steal rich ore, as from a mine by a miner. To mine only especially valuable ore (such as gold).

Examples:

• A miner highgraded the gold from the mine.
• The company decided to highgrade the mine to increase profits.

The term "highgrade" refers to the act of stealing or mining only the most valuable ore from a mine. This can be done by a miner who steals the rich ore or by a company that decides to mine only the most valuable ore to increase profits. The examples illustrate how the term can be used in both contexts.