Assaying is how jewelers and goldsmiths determine how pure gold is. It is the process of measuring the fineness of gold and there are various methods of doing so. Although there are many ways to assay gold, some are outdated and very inaccurate to say the least.
The most reliable method of assaying is the fire assay or cupellation method. This method has an accuracy of 2-3 parts per ten thousand (0.02%). During this method a very small fragment of the gold is taken and wrapped in lead foil with some added silver and heated in a furnace at about 1100 C which will remove all of the base metals. What is left is then pure gold, which is then reweighed. Assay laboratories use this technique across the globe for the purpose of gold hallmarking.
Some of the other methods that are used are more of a scientific process than anything else but many still refer to assaying as an art. Inductively Coupled Plasma Spectrometry or (ICP) is nearly as accurate as fire assay. A very small sample is taken and treated with acids. This is then judged but it is based on the color of the reacted area when compared to that of the reference sample. This method is very inaccurate and not used very often.
There is a newer method of assaying that is becoming rather popular and that is X-ray fluorescence (XRF). This method is non-destructive enough that it can be used not only for assaying gold in manufacturing, but can also be used to certify gold content in retail outlets. The only problem with this method is that the accuracy can be affected by the items being x-rayed; it is most accurate under perfect conditions. On imperfect surfaces the X-rays are often inaccurate or obscured.