Humans have adorned themselves and their belongings with attractive stones since prehistoric times. We’ve used fossil materials such as jet and amber, colourful rocks such as lapis lazuli, and water-clear single crystals of minerals such as amethyst and golden citrine.
The “precious stones” diamond, ruby, sapphire and emerald are distinguished from the remaining “semiprecious stones” largely on the basis of perceived rarity in classical times.
But what makes a stone a gem? It boils down to a few key qualities – beauty and durability. And rarity makes a gem even more special, as is the case for my favourite: painite.
Any stone may become a gem if it has beauty (in the eyes of enough beholders) and is durable enough to retain that beauty through everyday wear.
Durability usually implies that the stone is hard enough to resist abrasion from airborne sand and dust. Also, that it does not easily fracture or “cleave” on flat planes of weakness (determined by its atomic arrangement).
Diamond, the hardest known material, certainly satisfies the abrasion criterion. A diamond crystal does have four orientations of cleavage plane on which it can be split easily. But for diamonds, this apparent liability can be turned into an asset.
The cleavage is used as a short cut in the early stages of shaping, cutting and polishing this extraordinarily hard material, which is otherwise a slow and painstaking business.
The rare gem painite
Although diamonds are still the popular epitome of preciousness, they are far from the rarest minerals to have been used as gems.
As a mineralogist, my favourite amongst these ultra-rare stones comes from the gem gravels of the Mogok region in Myanmar. There, sapphires, rubies, spinels and other gemstones accumulate in river beds after washing down from the surrounding forested hills. These have been mined since ancient times.
In 1957, two deep red stones from a batch donated to the Natural History Museum in London – as shown in the lead image for this story – turned out to be completely new to science. A tiny slice from one crystal was used for research, and the new mineral was named “painite” after the original donor, the gem dealer Arthur Pain.
A third painite was identified in 1979, but it was not until 2001 that a fourth was found in Myanmar. Efforts to find more intensified, working uphill along creeks and locating progressively less water-worn material.
By 2005, a source outcrop for painite was finally discovered, nearly half a century after the original identification. Several thousand stones have now been recovered, but the small number of cut gems remains the preserve of specialist collectors.
Painite’s extreme rarity is due to it containing the chemical elements zirconium and boron, which do not normally associate with each other in nature and don’t occur together in any other mineral. Ironically, some painite crystals are partly altered to a crust of small pink crystals of the more common ruby.
The increase in supply means that you can now get small crystals of painite pretty easily online for tens of dollars, and poor-quality cut stones for about A$100. However, the tiny proportion of gem-quality stones still fetch US$60,000 per carat.
Source : https://theconversation.com/what-is-a-gem-and-why-painite-from-myanmar-can-fetch-us-60-000-per-carat-97453