What does this au750 do.30ct stand for and how much is it worth?

Au750 represents 75% gold content, namely 18k gold ring support.

D is the initials of diamond English.

0.30 carat stands for 30 points of a loose diamond (1 carat = 100 points).

There are 3,000 to 9,000 30-point loose diamonds, depending on color, clarity and cutting.