The SG of a mineral provides valuable information on the identity of a gemstone. SG seldom overlaps and is a fairly constant.
Firstly let us understand SG from a layman’s point of view.
SG is the comparison of the weight of a specific gemstone to the weight of an equal volume of water. The chart below gives a simple explanation.
The volume of One (1) cubic centimeter of water, at room temperature, will weigh 1gm. This is defined to be the density of water.
Gold has an SG of 19.3. That is, in a cubic centimeter of Gold (99.99% purity) the weight is 19.3gm or Gold is 19.3 times heavier than water.
SG for ruby is 4 while that of Jadeite Jade is 3.3. There may be some slight approximations due to the presence of other mineral compounds present inside but the variation is only very slight. These two minerals are considered ‘heavy’ as compared to other common minerals like quartz (SG of 2.57).
One of the method of determining the SG of a gemstone is by using a hydrostatic balance, as one need to weigh the specimen gemstone immersed totally in water. It can also be easily constructed. (If you want to do that just Google it or asked some high school students). A lot of jewelers used SG to determine the identity of a gemstone or to determine the content of pure gold in a gold jewelry.
Read more about it in my next post of using SG to do a ‘quick and dirty’ job of identifying a gemstone.