I'm not sure that's ever going to work out right.  You want any background to show, straight, through parts of the object; but in real life the light directly behind a gemstone would never shine straight threw.  It would get distorted, through refraction and reflection; dozens of times, before shining clear through the front side of the object.   As well as be mixed with light being reflected and refracted, from the other sides of the object, withing the gem. You'll really need to re-render the object for each environment(Sky Dome texture).

The best work around I can think of is to use a grey map on the sky dome texture.  Then, with Photoshop, clear out the extreme black and white values?