As you go to the higher standards the metal becomes yellower, denser and softer. This means that a piece of 18ct gold will weigh much more (1.6 times) than a piece of 9ct of the same dimensions (and so will be much more than twice as expensive).

In America 14ct is the minimum standard and this does, at least have the virtue of being more than 50% actual gold. In most European countries 18ct is the standard and they consider the 9ct that we use in the UK to be a pale and insipid imposter. You should consider the colour as well as the intrinsic value of the metal when choosing materials for your jewellery but please also be aware that, the higher the carat, the softer the gold will be and this may have a influence on what is practical for the design. 18ct gold rings often need to have claws replaced after a number of years of continuous wear and even the shanks wear out if they rub against anther ring on the same finger.

Hallmarking.

In the Uk there is a system of testing and marking the
metals so that an independent third party can verify
that what you are getting is the real thing.
For more information please see my Hallmarking page.

White, rose and green golds. (yes green)

The colour of gold can be altered depending on which metals are added to it in the alloying process. Oddly, a metal that is still 750 parts per thousand gold can be turned white by adding other metals. Get your children (or nieces) mixing poster paints and see if they can achieve the same result)
However, you should be aware that white golds are not white! (yes it’s a scandal isn’t it!) 9ct white gold is a very pale yellow and 18ct white is more of a grey.
To get around this, much white gold on the market is plated with rhodium to give it a hard white shine. By the time this has worn off you don’t look at the ring in the same way any more and will probably not notice the change in colour. I would generally choose not to use rhodium where it will wear off (but I will if you make me!) so you need to choose your white golds with care and an eye to the long term. If you want to combine yellow and white then 18ct golds give the best contrast and you should consider platinum or palladium if you want a better white metal.

You can also consider red or 'rose gold', very popular with the Victorians, and even green gold. To he fair, green gold is not that green but if you put it next to yellow gold then you can see a difference.

Platinum

Platinum is really expensive and really dense. It’s approximately
twice as dense as 9ct gold and about 5 times more expensive.
Therefore a ring that would cost £100 in 9ct gold is going to
cost nearer £1,000 in platinum. It is harder and whiter than
white gold and so you are getting something extra for your
money but you need to be aware that working in platinum
is a whole other thing financially and you might want to spend
the money on a bigger or better stone.....or something else entirely.

Palladium

Palladium is one of the platinum group of metals and comes just above it it the Periodic Table. It's very similar visually but is much less dense and much less expensive. It's been used in jewellery for centuries and was popular in Victorian times but it's experiencing something of a renaissance at the moment. It went something like this: There has been a trend, recently for diamonds set in white metals and so 18ct gold and platinum became popular choices for engagement and eternity rings. The, having been stable for decades, the price of those metals went bananas in the last few years and soared out of some people's price range. The platinum ring that was £600 was suddenly £1,300 and so palladium suddenly became a much more attractive option.

At the time of writing, palladium is about a third of the price of platinum and is also about half as dense. As the Americans say: 'You do the math'. You'll not tell it apart from platinum and the only possible down-side is that it can't currently be hallmarked. This will change, however, and the Assay Offices are currently working towards an agreement on standards and marks that should be introduced early in 2010.

Silver

Silver has always been much more common than gold and so is a fraction of the price. It's considered a semi-precious rather than a precious metal but you shouldn't dismiss it as a material for a special piece of jewellery. Because it is softer than the precious metals it's not really suitable for claw set engagement rings that will need to withstand a life-time's wear, but for wedding bands and more substantial pieces it's a good low-cost alternative.
The standards for silver are 925 (parts per thousand), known as 'Sterling silver' and 958 or 'Britannia silver'. Generally it's 925 that's used in jewellery as it's a bit harder and tougher than 958 and so will hold it's shape and resist wear rather better.

Skin reactions to metals

About two times a year I have someone contact me in the belief that I've sold them something other than silver because their skin had gone black....or green (usually just the finger, not the whole of it). Other metals such as copper and brass will do this but, once in a while, sterling silver has the same effect. It's usually to do with the specific metals that the silver has been alloyed with reacting with that persons specific skin chemistry and it's usually a ring that chasses the problem where perspiration builds up between the ring and the finger.

Because there are 75 parts per thousand to play with in 925 silver, different bullion manufacturers use different metals to provide different qualities for the resultant metal. Mostly it's copper but, especially when metal is being cast and it needs to flow and cool at the appropriate rate, there are traces of other metals in there as well.
It's generally one of these other metals that is causing the problem. This is why some people find that one 925 silver ring is fine but another one gives them a problem.

When this arises I generally try to remake the piece from a different metal so, if the original piece was cast, then I'll fabricate a new one
from sheet and wire which are processed in
a different way. If this doesn't solve the
problem then we can rhodium plate the piece
where it touches the skin. This will eventually
need to be repeated but it generally solves the
problem for a number of years. The only
other option is to have the piece remade in
a non-reactive metal such as platinum or
palladium and, although this involves more
expense, it may well be worth it if it's a
specialpiece that is going to be worn for years.