Teacher:You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.
Teacher: Now, understand the problem? Solve this instance: Category: STUPID ANSWERS 
Clue: This gemstone is basically an onyx with alternating bands of sard & other minerals
Student:
sardonyx