My favorite is ZALES. I get very good designs there and my wife loves it.

A few suggestions that may help you,

(1) Never take a chance with jewelry. Ask her.
(2) If you have to surprise her then you can also take her for surprise shopping
(3) Lastly, diamonds are forever.

I would also like to know if they back up the purchase with a Quality Guarantee Certificate of some kind and what kind of a return policy do they have?

I guess at times where a retail chain buys it diamonds can be a trade secret for they may have a preferable pricing over others. Sourcing of diamond has been linked to ethics, but then there are many other industries that import raw/ semi finished materials from places where child labor is involved....

There is no harm in traveling with your jewelry. But ensure that you keep it at a good safe place close to you while traveling. Rule of thumb: Never ever keep it in a check in luggage if traveling by plane.

Toothpaste cleaning is Ok to use but don't try it on costly jewelry. It may leave abrasions on diamond. From personal experience it works fine on silverware.

I have often seen Zales and other big jewelry places having discount sales on jewelry (including diamonds). sometimes the discount is upto 75%. Does it mean they misquoted the price to begin with or are they selling cheap jewelry? any thoughts!!

I will not buy jewelry on eBay. Off late there have lot of frauds and you can never trust the quality of jewelry until you see it in person.

I have bought from Zales retail stores and I am very happy with the experience so far.

There are not many big retail stores that keep gold jewelry in India. You may try out Big Bazaar, but you will not find a lot of designs. Best place is to look at small local retailers.