If you form a portfolio composed of a bunch of highly risky stocks whose prices bounce around a lot, the portfolio itself will not be especially risky if the price movements of each of the component stocks are independent of one another, because then the movements will on average cancel out. But if the returns on the stocks are positively correlated, meaning they tend to go up and down together, then a portfolio of volatile stocks remains pretty risky; the benefits of diversification conferred by holding a portfolio of the stocks are not as great.
