The terms "synthetic" and "simulated" are confused in the minds of most folks. Synthetic means a "REAL" diamond material but manmade. Simulated means a material that looks pretty much like a diamond but is made from something else. A "synthetic" diamond is actually diamond but is manmade in the deep secrets of the laboratory. The "simulated" diamond is either a CZ, a Moissanite or other simuilant.
So far to date, the moissanite is the best, even if color might be a bit off. Moissanite is a very durable stimulant for diamonds. The only drawback is the particular stone, since some may show a greenish tint. I suggest you ask to see more than one stone and compare the color for the whitest (least colored) moissanite.
Moissanite is a diamond like clear form of a substance you are likely familiar with: Silicon Carbide. Think of wet/dry sandpaper. That is silicon carbide, a very hard and durable material. When science was able to consistently manufacture clear crystals of silicon carbide, the gemstone "Moissanite" was born, named after the original discoverer from many years ago and before marketable crystals were producible.
This material is harder than most gemstones, not bothered by the heat used in jewelry work (meaning if prongs holding the stone need work in the future, the stone may be left in place for the work to be done). The stone is bright, durable, wonderful to the eye. The only distraction is the "sometimes" slightly off-color. That is why I suggest asking a jeweler to have more than one stone for you to select from. Do this with unmounted stones. Remember, carat is a weight and not a dimension!
Moissanite and diamond do not weight the same and a carat of one is not the same size as a carat of the other. Simply ask for a "diamond, carat, and size" when getting the stones to view. Most jewelers will do this automatically since they understand the size the customer wants is relative to the sizes of diamonds.
I believe a good look at some loose stones and a mounting selected and they you will be on your way, happy with the stone. These stones beat CZ is all aspects except the sometimes "tint" of color. CZ must be perfectly clean to really look diamond-like but the high refraction and characteristics of Moissanite keep it looking diamond like even when soiled somewhat.
Still, make an effort to keep all gemstones clean! All looks better that way and regular cleaning is the best way to check for wear and loose stones. Many jewelers will clean your item for free. Most should do that, I believe. Moissanite is an excellent choice. But, be sure to see the stone first and pick the whitest(most color free) stone.
"Metals." Platinum is an excellent choice for a ring but keep in mind that platinum will not keep a shine like white gold will. Platinum is durable, hypoallergenic, holds stones well and all that. Platinum is much more expensive than gold. Still, expect platinum do develop a dullish look after wearing a while. The other metal might be Palladium, another metal in the platinum family.
Palladium is a decent metal but many jewelers cannot work it properly and very, very little choice is on the market in palladium. We have "No" palladium rings in our business. Palladium is mostly used to add to gold to make a form of white gold. Now, a white gold made with palladium is an excellent metal choice.
You will have to go to a knowledgeable jeweler to find someone who knows the difference.This is what I mean: White gold is made by taking pure gold which is yellow, adding other metals to make it a "karat" gold, like 14k or 18k.(Pure is 24k and too soft for jewelry.) Sometimes metals is added to change the color of the karat gold. For instance, more copper makes rose gold. More nicklel makes white gold. In recent years, more manufacturers are using palladium instead of nickel since some people are allergic to nickel or have a sensitivity to the metal.
Those people cannot wear white gold made white with nickel. So, we have palladium white gold which is a slightly different white than nickel white gold but looks great and has no skin sensitivity problems. Some newer nickel metal mixes of gold meet higher standards and have no "sensitivity" issues either.
I suggest asking for white gold(if platinum is out of the budget) and finding out if it is palladium white or nickel white. Ask if the nickel white meets the new 'European standards' and is hypoallergenic. If the lady is use to white gold and has no problems with, regular nickel white gold should work fine. Just keep it out of anything with chlorine, like swimming pools, bleaches, hot tubs, etc. Chlorine works real danger on white gold made with nickel.
The wedding ring goes closest to the heart. In other words, put on the wedding ring first, then the engagement ring. In ancient times, it was believed that there was a vein that went directly from your ring finger on the left hand to the heart. That's where the symbolism of wearing the wedding ring on that finger started. The ring is not the most important thing. The love you two share is and that's what you need to focus on instead. I know he could be a little more generous with the price, but if that's his decision, go ahead and settle with it or make a compromise.