corundum, inclusions.5, gems

Sorby’s landmark paper demonstrates how microscopic examination of minerals and rocks can be used to reconstruct the conditions under which they formed. Using thin sections and transmitted-light microscopy, he studies crystal textures and, especially, microscopic cavities and fluid inclusions containing liquids, gases, and sometimes daughter crystals. Sorby shows that such inclusions may preserve samples of the fluids present during crystal growth and that changes in their liquid and vapor proportions upon heating can provide information on formation temperatures. From these observations he distinguishes minerals formed from aqueous solutions, molten material, and other geological environments and applies the method to interpreting the origin of rocks. The paper is foundational to microscopic petrography and fluid-inclusion research and is particularly important to gemology because the study of inclusions subsequently became one of the principal means of determining gemstone genesis, geological environment, and, ultimately, distinguishing natural from synthetic and treated gems.