Most people will recognise that the word carat has something to do with precious metals and gems, but will often be less clear about its specific meaning. Unfortunately the word carat has two different meanings, one that relates to the weight of precious stones, and the other that relates to the level of purity of gold and gold alloys.

Known as the karat in Canada and the United States, the carat is simply calculated as one carat being 1/24th purity of mass. Purity being measured by dividing the mass of gold in an item by the total mass of the said item. Thus an item made from all gold would be 24-carat and one made from only 50% gold would be 12-carat gold. The same basic calculations also ensure that a fineness scale can be achieved by dividing the mass of gold by the total mass of the item.

The millesimal fairness scale works as 999 (rather than the achieved 0.999 or above) as the equivalent of 24-carat gold, with 750 being 18-carat gold and so, the three digit number being a representation of the percentage of gold in an item. Going back into English history though a further measure of the carat was also devised based each carat being made up of four grains, and each grain being four quarts.

Knowing though what carat a piece of gold is, is different from being told what carat it is. If an item is displayed as 24-carat or 18-carat most people take it on trust that this is correct, after all testing is out of the question for most people.

Pure gold can be tested by undertaking a density test. Pure gold, or 24-carat gold has a density of 19.32gm/cm^3, something that can be easily determined by weighing an item and see how much water it displaces in a graduated cylinder of water. Density is calculated by dividing the mass of the item by its volume.

This test though can not be used on anything less than 24-carat gold though as the density is altered by other metals involved in the alloy, and it would be necessary to know the exact makeup to derive any meaningful results. For example yellow gold is made up of gold, silver and copper; white gold is an alloy of gold, nickel, zinc, copper, tin and manganese; and grey gold is made from gold and iron.

There are various testing kits that can determine the make up of a gold item, and can thus determine the purity and carat of that item. These testing kits though make use of acid and destroy a sample of the gold to get results.

Most purchasers of gold items therefore look to the assay or hallmark present on the gold item, although it should be noted that these can be faked.

Most gold pieces will have a mark on them like 18ct or 18K indicating the carat value of the piece. Additionally also put on the three digit millesimal fairness scale. Different countries have different carat or fairness values, below which an item is not allowed to be called gold, and only particular carats are allowed to be sold. There are though many benefits from being able to sell different carat valued items, as hardness and colours can be changed depending on the alloy, as well as lowering the all important cost of the gold piece. The assay or gold hallmark is a good indication of the gold makeup as long as the piece as come from a reputable source. It should though also be recognised that there is a tolerance associated with the manufacture of gold pieces in certain countries. Thus a piece marked 750 fairness in the United States may only be 747 fairness, which could affect the value dependent on the size of the gold piece.

For reference it should be noted that the metric carat when it comes to weight relates to one carat being 200mg, 3.86 grains or 0.007055oz, depending on the scales being used.

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