A diamond because when coal is pressured it turns into diamonds and only diamonds can cut diamonds.
Need more context please if you want help
Shakes, sounds, loud, soft, high, and low
The correct version of these two sentences when combined is:
When we were in Europe on vacation, we usually flew on airplanes: however, sometimes we took the train.
We apply the semi-colon to indicate that they have different ideas, and that they are clauses being joined into one sentence.