I would also suggest making sure you are looking at reputable antique vendors. Many times an antique diamond is not going to come with a cert. but I would probably try and have the ring looked at by an independent appraiser b/f you buy or have it written that the ring can be retuned if the appraiser doesn''t conclude that the price you paid is fair market value or was misrepresented in anyway.
Like many fancy shapes, an antique cut is tough to judge unless you can see it in person. Usually what I do if I''m wading through a big list of all types of quality- like Old World Diamond''s- is I tend to look at the depth and table size as a quick way to rule a lot of them out. I usually look for 60%- 63% depth, and tables under 60% (the smaller the better for tables IMO). Not to say that there aren''t some lovely antique stones that fall outside (sometimes WAY outside) those stats; it''s just that you need *some* way to weed through them! Also I look for an attractive pattern in the facets.
If you''re looking at them in person it''s easier. Basically I look for edge to edge sparkle, with no deadness in the center. A *lot* of antique stones have center facets that just kinda... sit there. It''s not always as obvious as, like, a darkness under the table- it''s just that you will see a lot of them that the fire only happens in the outer facets and the chunky center ones don''t do much at all. And of course an attractive facet pattern.
Also look for chips very carefully with a loupe, and then decide if they bug you if you find them. Often antique stones will have small chips you can only see with a loupe. While that will affect the clarity grade, if you can''t see them with the naked eye personally I''m usually OK with one or two minor dings on the girdle. If the price is right.
Definitely get a return period if you''re buying one online. And have an independant appraiser *who is familiar with antique stones* look at them. If you have a stone that is uncerted and is only graded by the seller- a lot of antique stones are- you might find the color overgraded. A lot of antique cuts face up whiter than they actually are.
Old Mine cushion cuts and Old European cut round diamonds can run from virtually Tolkowsky type measurements to ultra thin and to extremely deep stones and everything in between. You must look at the parameters to know the nature of the stone and then you must look at it in person, if at all possible, just to be sure it is a beautiful rock to your way of thinking. Also, old stones are frequently chipped, or abraded. Make sure you know the condition of the stone or have an expert check it for you. A couple tiny chips or nicks may not mean anything, but before you spend your money, be sure.
Also, I see a lot of reproduction mountings, made much more recently, that mimic antique mountings. Some of these are very nice, but some are simply junk. You need to discriminate between acceptable mountings and ones which are already too thin or worn away at the time of purchase. You don''t want to pay $1000 for a worn out ring or for a new ring which is being touted as "antique". Many sellers use "antique" and "antique style" in a way to confuse or cloud the issue. "Style" means it isn''t old, just looks that way. Also, rings from the 1950''s to 1980''s are NOT antiques. They may be "estate pieces", 2nd hand items, but they are not even 80 let alone an antique age of 100 years old.