How price of cryptocurrency grow

One person asked me: how is it happens that sometimes price grows and sometimes become less?

Imagine that John have bought 100 diamonds on market 10 bitcoin each. He invested 1000 bitcoin total. Now he wants to make more bitcoins.
He opens his own exchange d.com and tell his friend Peter that he can buy diamond there only 11 bitcoin each.
Peter buys 50 diamonds from John giving him 550 bitcoin total. He also want to make more bitcoin and call his friend Tom and say that he can invest his bitcoins in this diamonds.

Now Peter and John decide that they can sell diamonds only 12 bitcoin each.
Tom pays this price and also want to make money, that is why he calls his friends Sandy, Michael and Ivan. And so on...

I hope now it is clear that the more people want to buy the more price grows. and opposite: the more people want to sell the less price become.

Same things happen with any good in any exchange.