I'm going to add a little more to what previous poster stated.....
I have been working in the jewelry industry for 10+ years
white gold - gold is yellow so there really is no such thing as "white" gold. They basically are adding more white alloys to tint the gold a lighter color. Gold comes in different weights - 10kt, 14kt, 18kt, 24kt. 24kt is pure gold while the others is how many parts gold to parts alloy mixed - 14kt is 14 parts gold/10 parts alloy. White gold jewelry pieces are finished with a chemical called Rhodium - it is was gives white gold pieces the high white polished finish - it is a liquid that adheres to the gold and not the stones. How long that finish lasts depends on the wear of that piece. Rings will wear faster because they get more contact abuse - most of the time the wear shows on the bottom more because you are hitting that spot more. I have found in my experience that most women re-dip or re-rhodium their jewelry about every 6 months to yearly depending on the person (cost is about $35 per dip). Older jewelery had different alloys mixed into it so the white gold pieces stayed white....they do not use the major alloy that they used to use in making white gold pieces because of availability/expense. So if you are speaking about grandma's ring then it probably was made with a different mix of metals than today.
Platinum is a denser metal - it is stronger, however because of that it scratches easier - instead of tinting another color it dulls to a matte gray finish. It is also heavier and roughly 3x the expense of gold settings. Some people just do not like platinum. To refinish a platinum piece it can be buffed back to new condition. If you have allergies - nickel for example - platinum is a great way to go because generally speaking a purer metal - in order to be stamped platinum it has to have a certain % of platinum in it. Some stores do a platinum blend, you have to ask what the percentage of platinum they are using. The pieces we sell in the store I work at are 99% pure platinum. Some stores are as low as 60% I believe. As mentioned previous it is pretty common to do a mix - platinum head (or prongs) and white gold (or even yellow gold) band. If your setting for example only has 4 prongs to hold the diamond in place I would recommend platinum because of it's strength/durability.
As for sizing - platinum must be laser cut because of it's strength. Most jewelers do not have the right equipment on site to properly size/buff these pieces so your turn around time to being back in your possession is longer. Make sure they are using platinum solder when they are making repairs or sizings! Some stores will size with white gold on site but send out to do it right with platinum. White gold is easier to have sized because most jewelers can do on site and quicker turn around time. However make sure they can also re-dip the piece at the same time otherwise you may see a coloring difference where it has been altered.
As for the diamond itself....many diamonds are certified by many different companies - all diamonds are graded not all diamonds are certified. Your most popular are GIA, AGS, EGL but there are many, many companies out there. GIA & AGS are the 2 certification companies that have the strictest standards. What I mean by that is when they grade the diamond - color, clarity, etc. they have a high standard to meet those criteria. Alot of those diamonds are laser inscribed - what that means is there is an actual certification number imprinted on the edge of the diamond (called the girdle). There are a couple of good things about laser inscribed diamonds....One, if you have work done on your ring you can look under a microscope and make sure it is your diamond you are getting back and Two, should something happen to your diamond and it is insured then the insurance company has to replace it with the same thing - there is no doubt that if you had a G colored, VS2 clarity diamond certified by GIA then you will get a replaced diamond that is G colored, VS2 clarity certified by GIA.
Good luck & CONGRATULATIONS
