Remember, all diamonds are not created equal. Two diamonds of equal Carat Weight may vary substantially in price due to their Cut, Colour and Clarity. The carat weight of antique engagement ring is a very important determining factor in their value.
While the weight of ideal cut is important, it doesnt necessarily provide the "perceived size" of diamond necklace. A shallow cut diamond will have a greater diameter, and will therefore look larger.
The value of diamond earrings is determined by theirs grading under the 4 Cs, color, clarity, cut and carat weight.