I get this question a lot, since people naturally assume that pure gold would be preferred to impure gold. The truth of the matter is that, while pure gold has a highly desirable color, it is generally too soft for most jewelry.
For those of you who aren't familiar with gold ratings, gold is typically rated on a karat scale, specifying the total proportion (by weight) of gold in the alloy. The scale is out of 24, so 18 kt gold contains 18 parts per 24, or 75% gold.
The remainder of it can be alloyed with a variety of different metals to achieve different results. Silver and copper are most common, but nickel or platinum can be added to achieve white gold. The proportion of copper can be increased to achieve rose gold. Numerous other addition metals can be added for a variety of reasons, like increasing hardness or improving the castability of the metal.
To me, 18kt gold provides the best balance of working and wearing characteristics with brilliant gold color, but 14kt is also very popular (and slightly less expensive).
18kt is soft enough to be worked cold in my studio without constantly needing to heat it up with a torch, but when completed, can be worn daily without major wear -- preserving the finish and ensuring stones stay set (a polished finish will eventually fade slightly to a satin finish after a year or so, so that is why I typically recommend satin finishes in general).
In America, you do not typically see much jewelry worn beyond the 18kt purity. However, gold can be alloyed in any proportions, so 20kt, 22kt, and 24kt gold are used in practice from time to time.
Jewelry made of 20kt gold will inevitably acquire scratches and small dents after a year or so of daily wear. Additionally, it does not hold onto a polish quite as well as 18kt. Some people actually like this feature of 20kt gold for wedding bands, since the dings and dents represent the struggles that couples overcome as their relationship grows and develops.
20kt might hold a stone, but I certainly would not recommend risking it. I can make jewelry in 20kt if you understand all of the risks associated with that, and can commit to the level of care necessary.
22kt is very rarely used for jewelry, and when it is, it is typically used for show pieces that are worn very rarely, and polished before or after use. This practice is far more common in Asian countries, like India or China than in America. I generally do not make wearable pieces in 22kt, but do from time to time use it for figure work.
24kt is simply too soft for jewelry of any kind. It is soft enough for you to leave bite marks with your teeth. 24kt is typically reserved for gold bullion as a store of value only. These bullion pieces might feature designs or ornamentations, but they tend to be simple relief work.
The brilliant gold color of 24kt is unbeatable, however, so I am always experimenting on ways to use it in jewelry where it can be protected by harder metals -- it's one of my passion projects.
So yes, if you really want it, I can make you a 24kt ring, but I'm not going to guarantee it for maintenance! 😂