Emeralds always have a balance between clarity and returned light, the sparkle that is. You do want a stone with at least a decent amount of returned light. What I mean by this is light that goes into the top of the stone and is reflected back out to the eye. Otherwise, the stone looks rather dead and even the color is not as apparent.
Some light coming back to the eye is needed to bring that color to life. There is a trade off often between color and brilliance, it seems, unless the stone is completely top gem quality and has both color and clarity at the same time, accompanied by well proportioned cutting of the gem.
"Dull Look On The Surface". Why would one stone look like it needs to be "wiped off" and another look smooth and well polished? Generally, this is because the well polished stone is just that, a piece of emerald able to take a good polish and then polished well by the cutter.
The duller or more waxy looking stone is either less well polished in the first place or the emerald material simply cannot take a better polish. Why not a better polish? Defects in the stone are a general reason. Often, well, almost always, emeralds are treated to some extent and that is generally with oils which tend to hide inclusions.
The inclusions which reach the surface of the stone are pretty well hidden by oils but this can also prevent a perfect polish. Unfortunately, over zealous cleaning of an oil treated stone may remove the oil and suddenly you see cracks and furrows not visible before. That is the stone as it was before being subjected to oiling! You might want to ask the jeweler about oiling on the cushion stone you have seen. Do not be surprised to hear the answer is "most likely" or "yes".
Since most emeralds are treated in this or similar manner (some really nasty ones are given a plastic sort of filler!), do not be dismayed. Only be certain the jeweler can stand behind the stone, especially considering the price of the one you like for your setting.
"Jardin". The inclusions in emeralds are called a "garden" or "jardin". This refers to the almost garden look to the flaws and swirls inside the stone. This is to be expected in most emeralds of any size, with more or less garden. Inclusions are expected. What to watch for are definite internal fractures, little breaks which appear healed with in the stone.
Such internal flaws near the areas where the setting applies pressure, as with prongs, may be dangerous in the setting process. Ask to see the stone under a scope and ask if any of the inclusions will lead to stone setting problems. At this point, all risk is in the hands of the jeweler doing the work but ask to be sure!
"Memo Goods". A reputable jeweler with connections to the colored stone world can receive what is called "memo good" from stone vendors. The vendor will send a small selection of gems for the customer to view. You might be responsible for the postage but should not be considering the purchase prices overall. The jeweler can check regular stone vendors they work with or go on the net to specialized jewelry web sites where stones are requested and sold everyday. The jeweler might be able to find a cushion cut for the setting you prefer and find a better stone in the process.
If you have a little time, this might very well be worth requesting. Do not feel locked into what the jeweler has in stock. Other stones are out there and should be available to an established jeweler. The cushion you like might be just fine, of course I have not see it. By all means, if you have any doubts about the gem, look further. You do not want to spend the money and always have a little feeling that you should have gotten a better emerald.
That is why I tell everyone that if you look for one or more settings that you like you could always get the one that you want for the inter net, jewelry store close to home, or a jewelry store that is out of state. Don’t take what you get see what they get and then make ulp your mind on what you want.