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Specific Gravity

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Determining SG Using A Weighing Scale - Hydrostatic Balance

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.

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I am a Graduate Gemologist trained at the Gemological Institute of America (GIA) in New York City, USA. I hold an MBA degree from Cranfield University, United Kingdom, and a Bachelor degree in Mathematics. My earlier profession was a banker until I found jade in Myanmar (Burma) in the early 90s. I have traveled to the fabled Hpakan Jade mines, and Mogok, the world’s famous rubies and sapphires mines in upper Burma, with my second son. Three of my children are also Graduate Gemologist, GIA, NYC and they deal in diamonds, gemstones and jade. 我是在美国纽约市的美国宝石学院(GIA)接受过培训的宝石研究学家。 我拥有英国克兰菲尔德大学的工商管理硕士学位和数学学士学位。我以前的职业是银行家,直到90年代初我在缅甸接触到玉石。我曾经和我的次子一起去过缅甸上流传说中的哈帕翡翠矿山和莫谷矿山, 莫谷矿山是世界上著名的红宝石和蓝宝石矿山。我的三个孩子都是纽约市GIA毕业的宝石研究学家, 他们专门处理钻石,宝石和玉石.

35 COMMENTS

  1. Hello Asif, The SG for a ruby or a sapphire loose stone is very accurate. If you can get it around 4 the it is almost sure that the stone is a corundum. You may need to improvise some equipment to calculate the SG. First take the weight in air, then take the weight in water. By dividing the wt of the stone / wt of the stone in air – wt of stone in water, you will get the SG.

    Ask any high school students and they will be able to guide you. thanks,

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