Phase Transformation of Epigenetic Iron Staining: Indication of Low-Temperature Heat Treatment in Mozambique Ruby

Abstract

In the past several years, Mozambique has emerged as one of the world's most important sources of ruby, and unheated stones from this country are in particularly strong demand. Nevertheless, it is common for these rubies to undergo low-temperature heating (~1,000°C or below) to slightly improve their colour. The treated stones may show very subtle or no alteration of internal features (e.g. mineral inclusions, 'fingerprints', needles, 'silk', etc.). However, 'iron-stained' surface-reaching fractures in the rubies commonly display a noticeably more intense colour after heating. Raman and FTIR spectroscopy were used to document a transition from goethite to hematite within stained fractures in samples heated to 500°C and 600°C. The identification of hematite within such fractures provides key evidence for the low-temperature heat treatment of Mozambique ruby.

A green approach for treatment of wastewater with manganese using wood ash

“…. The specific peaks of the main functional group are: 3696 and 3648 cm −1 for OH stretching attributed to the inner‐surface hydroxyl groups, 2873–2970 cm −1 for organic carbon, due to the incomplete burning of wood and 2517, 1795, 1448, 1067, 876, and 712 cm −1 for vibrational modes of CO groups from carbonate …”

Low-Temperature Heat Treatment of Pink Sapphires from Ilakaka, Madagascar

“…After treatment, orange epigenetic iron stains in surface-reaching fissures became darker due to the oxidation and dehydration of iron minerals at higher temperatures (figure 3H). Typically the orange iron stains consist of limonite (α-FeO(OH)) and convert to brownish red hematite (α-Fe 2 O 3 ) during heat treatment (Koivula, 2013;Sripoonjan et al, 2016). The other observed change was seen in monazite crystals, which changed color from orange-brown to near-colorless after heat treatment.…”

“…In contrast, heating corundum at low temperatures, sometimes below 700°C, will only subtly affect their internal inclusions, making the treatment much more challenging to detect without advanced instrumentation (figure 1). Previous studies on the effect of low-temperature heat treatment on inclusions have focused on rubies from Mozambique (Pardieu et al, 2015;Sripoonjan et al, 2016;Saeseaw et al, 2018) and blue sapphires from Madagascar (Krzemnicki, 2010;Hughes and Perkins, 2019). In these studies, rubies showed slight inclusion alterations when heated to 900°C and developed clear…”

A Decade of Ruby from Mozambique: A Review

“…The platelets never showed alteration at 600°C, but under the conditions of this experiment the stone's color was not altered (Saeseaw et al, 2018). Fractures with iron staining showed a change from yellowish to brownish red, even when heated at low temperatures (Sripoonjan et al, 2016). However, these features are not commonly seen in faceted stones.…”

“…Many Mugloto-type stones show this subtle color alteration, which can be removed by heat treatment (Pardieu et al, 2015;Sripoonjan et al, 2016;Saeseaw et al, 2018).…”