1. What is the hardest natural substance known to exist?

2. What is the thermocline, or transitional, zone?

3. What makes a gem "precious"?

4. Of what substance are clouds comprised?

5. Coal, gas and oil are all organic materials from different sources of matter. What is the basis from which these energy sources evolve?

Answers

1. Diamond.

2. The thermocline is the area in the water depths, where water, heated by the sun, suddenly changes from warm to cold. There is no gradual cooling.

3. Gems are classified as "precious" if they are naturally occurring stones.

4. Clouds are formed from minute drops of moisture or particles of ice.

5. Gas and oil are derived mainly from animal matter. Coal comprises plant remains.

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