GEM MATERIALS: RESEARCH INTO THEIR ROLE IN THE EARTH SYSTEM

Gems are among the most beautiful physical products of Earth’s geological processes and are revered in all cultures. The surprise is that scientific research into how the Earth system creates them has a long way to go, except perhaps in the case of diamonds. Fortunately, geological research on gems and gem deposits has blossomed in recent years, in part because the topic fits well into education where beauty, interesting stories, cultural connections, and scientific understanding can advance knowledge and appreciation, particularly of students. The vantage point from research carried out at the AMNH provides examples.

Jadeite (jadeite jade) is the prize of Southeast Asia, but its geological origin had been enigmatic until less than 30 years ago. Jadeite (NaAlSi₂O₆) was appreciated as a high-pressure mineral from about 1950, but the needed conditions and monomineralic crystallization required understanding of the implications of plate tectonics and the role of hydrous fluid in subduction channels. Moreover, jadeite jade sources were apparently very rare, actually overlooked if not of gem quality, until the geological formational process was better understood. Now, its origin in plate tectonics is better comprehended, and its capacity to record fluids expelled from subduction channels is important to science. In addition, the mineralogy and geochemistry of archaeological jade can be compared to that from sources to understand trade routes of our ancestors.

LA-ICPMS has proven extremely useful in studying the trace element chemistry of gem minerals. Data from ruby and sapphire have helped define origin from marbles vs. clastic sediments vs. igneous xenocrysts, although studies must continue to better understand crystallization processes. In one case, Mogok ruby, local variations may relate to sourcing from black shale pods with different chemical signatures, whereas secondary ruby records metasomatic growth in skarn resulting in high Si content in the crystals.

Study of large peridot (forsterite) crystals from three exhumed dunite bodies has shown these crystals grow from hydrous fluids in tension gashes (pockets) at conditions where olivine and fluid can coexist. This origin is yet another indicator of the important role of plate tectonics in gem formation that has ramifications for the Earth system.