Cushion Cut Diamond Cut Quality: Depth, Table & Avoiding Crushed Ice

Question
Hi, I'm interested in a square-ish cushion cut diamond, about 1.5-1.6 cts, but am having a hard time figuring out what's considered a good cut since there seems to be no established consensus on that. I don't like the "crushed ice" look, so are there depth and table %s to stay stay away from to avoid that look? I recently considered a 1.57, 1.25 ratio (I find that too rectangular for my taste) with table 59% and depth 67.7%. I feel it's a a little bit "crushed ice" but otherwise can't tell how good this cut is (it doesn't look too different than other stones I saw).  I see alot of diamonds online with these similar table and depth %s. The GIA doesn't give other measurements, so when a jeweler tells me that all these other numbers are important too, I feel quite lost.  Can you give some guidance as to what to look for in a stone this size to get a nice clear symmetrical look that has depth to it when you look into the stone?
Also, are other certifications reliable too or should I only look for GIA? And do you think an IGI appraisal is necessary before I buy or will a jeweler's own appraisal do, for insurance purposes?  
One more question please: do you think 18K white gold is a problem as far as the safety of setting a diamond in it? should the prongs be platinum or palladium or better not to mix metals?  Thanks so much, I know it's alot!.

Answer
Rania
Lets try and take these one at a time....
not sure what you mean by crushed, unless you mean a salty white crystal look because the facets reflect into a corner and may look like white scratches??? Table and Depth percentage should be 57-60% with a depth of 61-62% for ideal round stones.
Cushion cuts are not too far off these as well, the ratio should be close to even but very difficult to find and you will pay extra for better proportions. call me 888.293.8881 and I will check to see what I have for you on Monday.
The GIA and EGL are the only recognized certifications or Lab Graded reports in the industry, they can also provide for a dollar amount appraisal but usually are full retail pricing. I need to know what color and clarity you desire to give you a price.
18KT is a sales tool used to show customers specialty of a ring, its too soft for a diamond engagement ring as you wear it all the time, the prongs should be either white gold 14kt, Platinum if you can but a little more expensive. I would go with 14kt as there is NO noticeable difference in the two.
all the best
Les