I'm sorry to say but it's not possible to "make" diamonds at home... The fancy colored diamonds you buy at stores are expensive because they're so rare and a lot of work was done in order to make them look so shiny and clear.
How Do You Make Homemade Diamonds?
You need a cup,pencil,salt,and food coloring put water in the cup and the food coloring then tie the string to the pencil and put the pencil on top of cup were the string would be in hhe water and then wait 1-2 day sand the string will turn into a Diamond :] try it!