Sapphire secrets: they aren’t all blue, and mining them requires luck plus labour – by Lynda Lawson (The Conversation – August 8, 2018)

Lynda Lawson receives funding from Tiffany and Co Foundation and Deutsche Gesellschaft für Internationale Zusammenarbeit GmbH (GIZ) , a German development agency.

I first remember seeing sapphires as a teenager in a jeweller’s shop in Silver Street in pre-Khmer Rouge Phnom Penh – the deep saturated blues of the gems from Palin on the border with Thailand were captivating. The sapphires my father bought that day are still in the family long after any trace of Silver Street has disappeared.

It was not until recently when I met a female sapphire miner in Madagascar that I began to appreciate the hard labour involved in the mining of these stones across Africa and Asia. Sapphires are crystals of the mineral corundum, made up mostly of atoms of aluminium and oxygen in a 2 to 3 ratio (Al₂0₃).

The chemical bonds of aluminium and oxygen are particularly tight, making the sapphire one of the hardest minerals known – 9 out of 10 on a measure of hardness used for minerals known as Moh’s scale. Sapphires are second to diamonds in hardness.

This quality makes the stone valuable for jewellery, as it keeps its shine and does not scratch. The aluminium and oxygen do not reflect light, making sapphires transparent, another characteristic that is highly prized in a gemstone.

The tight chemical bonds of corundum also make the stones very dense and relatively heavy. This means they are not swept away during weathering, but accumulate in alluvial deposits – that is, left behind by the flow of water. As a result, sapphires can be found by miners tunnelling down to ancient riverbeds or in bends of existing river beds.

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