To get the marble you have to buy some flower seeds and plant a moon drop flower and a pink cat flower then when they are done a boy comes to the farm (Stu) and tells you to plant more so then plant four more moon drop and four more pink cat flowers then when there done the boy comes back and ask you to trade with you and say yes then you get a marble (you can find pink cats at the Flower Festival and buy at least five of it).