There are a few factors you need to consider when you choose the setting for the ring. Yes, the design is an important factor. You would definitely want to choose a ring that suits your taste. However, there are other equally important factors you need to think about, which are listed here.
The Ring Design
In taste, there is no dispute. Choose whatever design you like. You can opt for a simple platinum setting like my proposal ring, or a titanium tension setting. They are all lovely. However, do take a few practical points into consideration.
Firstly, you should choose an open setting (like a 4 prong setting or a tension setting) only if your diamond is of a high colour grade. The colour of a diamond is most visible through the pavilion, even if the diamond is well cut. If you must have an open setting, I would strongly suggest that you pick a diamond with a colour grade of "F" or better.
Next, if you would like an open setting, make sure that the pavilion of the diamond is eye clean. Even VS grade diamonds can sometimes have eye visible inclusions on the pavilion.
If you like a tension setting like the titanium one one seen here, do not wear it while you are doing manual work. The ring is extremely beautiful, and it may be made much tougher than even the hardest prongs. However, note that impact on any part of the ring, would affect the integrity of the setting. For traditional prongs, we are only concerned with the integrity of the prongs. For example, you may hit the ring even when you are opening a door, hand clapping against a ring in another hand, etc. It would also be a good idea to insure such a ring because it there is a limit to how sturdy tension settings can get.
Hardness is measured by the Vickers scale: HV
This is the measuring the penetration of a object into metal at a determined load. This is not strength, just hardness.
Harder metals tend also to be more brittle. Ones that are softer scratch easier, but are a bit more forgiving to slight bending in use and seem to hold onto their stones a bit better.
Also, the hardness of the metal can be a consideration in the design process. Filigree or open work type items need hardness to be durable. Heavy, thick items need less hardness as their durability comes due to their weight and thickness.
Gold and platinum are soft metals and their alloys control much about their hardness. I will discuss the hardness and other details of White Gold and Platinum in the next couple of pages. However, the hardness of these alloys can be altered by work hardening.
During custom work, die struck rings are subject to knocking and hammering to achieve its shape. This process of knocking hardens the alloy considerably. Unfortunately, when the stone is being set, the prongs need to be annealed (heated up and then slowly cooled). Annealing softens the alloy. None-the-less, it is agreed that die struck settings are harder, even after it has been annealed.
If the ring is die cast (pour molten alloy into a cast), it does not benefit from work hardening. Also if micro air bubbles are introduced during the casting process, the density of the alloy is further decreased. Hence, quality control during the die casting process is important.
Custom or Ready-Made
Another decision you have to make, is whether to purchase a ready-made setting, or custom make one. This decision probably hinges on whether you can find a ready-made setting off the shelf. If so, you are in luck. However, you may consider custom settings for that extra character.
Ready-made settings are simple. They are mostly die cast. The only thing to worry about is the hardness of the alloy, and how the diamond is actually set in the prongs.
If you do choose a custom setting, it is important to find a good craftsman to fashion the ring for you. Some chain store jewellers retain some goldsmiths to do custom work for their clients, but their skills vary. If you happen to use a bad craftsman, be ready for a nasty shock, because the outcome can be very bad.
When custom making a setting, expect some degree of variance from the design or sketches you presented to the jeweller. Custom work is a bit like painting still-life. You can get close, but you can never reproduce the original design 100%
Do budget enough time. Do not rush the goldsmith. When the ring is done, inspect it thoroughly. There is bound to be some flaws for custom work, so do not nit pick. However, there is no reason why you should accept blatantly shoddy work. Let the diamond setter set the diamond only if you are completely satisfied with the setting because taking the diamond out for alterations later will weaken the prongs.
Be prepared to sacrifice the deposit you paid for the ring. It is better to lose the deposit than set the diamond in an ugly ring.
You may wish to read this article from the Business Times regarding custom making a ring.
You will not have a problem with whiteness if you choose a platinum ring because it has a very white look naturally. It can be scratched or polished many times, and still remain white. Platinum has a very slight greyish tone though. Some people like to give their Platinum rings a Rhodium plating for that extra shine, but this is a matter of taste (and not entirely necessary).
However, if you opt for a White Gold setting, be aware that it is not white. White gold is usually slightly yellowish (depending on the alloy content). Which is why Rhodium plating is necessary. It is the Rhodium plating that gives the white gold ring its white look. However, Rhodium plating wears out after a couple of years, and the yellowish tone of the actual White Gold will show through. By which time, you will need to get the ring Rhodium plated again.
All rings get scratched. Some people choose certain alloys, thinking that its hardness will reduce scratching. That is just being silly. It makes more sense to wear the ring proudly. If it gets scratched, you can easily polish it for a small fee.
When Platinum rings get hundreds of scratches, you will not be able to see the scratches for the scratches. That is called a 'patina', which is not at all bad looking.
The same cannot be said for White Gold. When White Gold gets scratched, it loses its Rhodium plating and will appear slightly yellow. Polishing a White Gold ring involves putting on a new coat of Rhodium plating. For Platinum, it is not necessary, although some people like it.
Both White Gold and Platinum can withstand many polishes, far beyond the life of most people. So, please do not worry about scratching your ring.
Some people are allergic to Nickel, which is used in most White Gold settings. If your girlfriend is allergic to Nickel, please don't propose with a White Gold setting unless you are sure that Nickel is not used in the alloy content.
Allergic reactions range from minor rashes to cold sores. It is not pleasant at all.
If you are unsure, I suggest you opt for platinum.