The orginal poster (I believe) was not asking about a half of an eighth, he left out a comma between half and eighth.

eighth -3.5 g
quarter -7 g
half 14 g
ounce 28 g

and by the way (weigh), 3.5 in an 8th is not an "industry standard", an 8th is 3.5 g because 3.5 is an 8th of an ounce.