What is the composition of Olympic medals?

Question

Are gold medals made of solid metal or plated? If solid, is it pure gold or an alloy?

The same question for silver and bronze. I noticed the medal size is quite substantial. Please contribute only reliable information - no guesses.

Answer 1

The gold medal is 72% silver and worth about $300.00!

Answer 2

For the first time in Olympic history, the medals for the Beijing Games blend metal and jade.   each medal is plated with 6 to 6.1 grams of gold. ......each medal must be at least 2.4 inches in diameter and 0.12 inches thick (three millimeters thick and 60 millimeters in diameter)