A large new diamond mine has just opened and the price of diamonds has gone down since she has study economics is Cecilia decides to buy diamond necklace and she knows that

The answer is:

In this case, Cecilia only decides to buy the necklace, knowing that the cost of diamonds depends only on the demand.

She will find a good price, because she knows the product availibility exceeds the demands, making the prices go low! when people start looking for this kind of diamond, the reverse will happen. The demands will exceed availability and prices will rise. Cecilia takes advantage of the fact that demand is low.