loose diamonds is more commonly found in certain shapes; baguette, emerald, heart, marquise, oval, pear, princess, radiant, round and trillion.
Dont be fooled into buying jewelry that is said to be .25 points. It is NOT a quarter of a carat, and will be so small that it should only be used as accent to other stone or the gold itself.
While the weight of diamond engagement ring 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.