although water is useful yet its cheap on the contrary diamond is very expensive. give reasons for this paradox

Price of product is determined by supply and demand force of market.  If  Qs (Quantity supply of x goods ) is> that Px (price  of x commodity then price will be low and vise versa Qs(x) is<Px then price will be high  .In water are available  in plenty  as compare to it demand  so price is low or it is cheap though it is very useful. and diamond is very expensive because it supply is very scare.. suppose you are in desert  and you have  4 diamond with you and you are very thirsty  and you have no water than perhaps  you maybe ready to give 4 diamond  for getting one bottle of water .