New High-Quality Rainbow Moonstone from Madagascar
Moonstone, a perennial favorite in Tucson, was offered both as loose stones and in jewelry throughout the shows. This year, a new type of transparent moonstone from Andilamena in the Alaotra-Mangoro region of northeastern Madagascar displayed a distinct orange flash effect in addition to strong blue adularescence. Moonstone is normally cut en cabochon, but the two moonstones in the figure above were faceted because of their high transparency and to better display the orange flash.
Blue moonstone is a phenomenal gemstone variety of orthoclase, a mineral belonging to the alkali feldspar group. This potassium aluminum silicate gem is transparent to translucent, with a white bodycolor and blue adularescence. The blue “moonlight” is caused by light interference from moonstone’s alternating microscopic layers of albite and orthoclase. Rainbow moonstone, which shows flashes of various colors that can include blue, is also a feldspar mineral but belongs to the plagioclase group and falls into the calcium-rich labradorite variety. This new material was confirmed to be plagioclase feldspar and within the labradorite variety by Bear Williams (Stone Group Laboratories, Jefferson City, Missouri) using Raman spectroscopy, making this a high-quality rainbow moonstone.
This new material was reportedly mined in 2023, and the initial supply of several kilograms of rough sold out at extraordinarily high per-carat prices. Hopefully more of this intriguing material will reach the marketplace soon.
