The Girlfriend's Guide to Jewellery

by Anne Wallner Rss
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15: Density

When we were kids our dad used to tease us asking ‘what is heavier, a pound of lead or a pound of feathers?’ of course the answer is that they both weigh the same. What is different is their relative size or volume. A pound of lead is pretty small and a pound of feathers is huge! This is a lesson in a contrast between two materials each of vastly different density.

Different gemstones have different densities. Density is referred to or measured as the ‘specific gravity’ of a material. The ‘standard’ for measuring density is the weight of one cubic centimetre or 1cc of water. At sea level and at 4 degrees celcius one cubic centimetre of water weighs one gram. Water is our ‘benchmark’ and it has a specific gravity of 1. A volume equal to 1cc of diamond is 3.45 times as heavy as 1cc of water. Diamond has a specific gravity of 3.45. Sapphire is 5.43 times as heavy as water and has a specific gravity of 5.43. What we see is that sapphires are actually heavier than diamonds, 1.57 times heavier.

Why do we care? Say for example we see a three stone diamond ring with all three stones the same size, all round, each measuring approximately 6.5mm in diameter, each weighing one carat and having a total diamond weight of 3.00 carats. We like the ring only we want it set with three round brilliant cut sapphires. Three sapphires each having all the same measurements as the three diamonds will have a total weight of 4.72 carats, 1.57 times heavier than the three diamonds.

Depending on how you phrase your request, you will end up with two very different rings. If you use weight as your measuring stick and ask for a three stone sapphire ring with a total weight of 3.00 carats each stone will measure approximately 5.5mm in diameter. The sapphires will be smaller but the total weight will be 3.00 carats. If you use size as your measuring stick and ask for a three stone sapphire ring with each stone measuring approximately 6.5mm in diameter and the total weight will be approximately 4.72 carats.

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