3.5 grams is an Eighth (Eighth of an ounce), 7 grams is a Quarter (Quarter of an ounce), 14 grams is half an ounce, and finally 28 grams and some change is an ounce. If you are asking what kind of item you can play that will equal an eighth of an ounce of weed; there are several. A bic pen cap or a single Smartie candie equals pretty damn close to a gram; maybe like .98 or .99 grams. A US nickel is 5 grams. You can always buy little weights and any craft store though.
Don't worry about the crappy ghetto weed though, that's all I can usually find. Still gets me stoned though so it's all good. As soon as I get my own spot, I plan to grow several plants =D

EDIT: They call it a quarter because it's a quarter of a dollar. (4 quaters = 1 dollar). Buy anyways, to equal a quarter ounce, put a new US nickel and 2 Smarties = 7 grams.