Currently, semantic interoperability for documents delivered by the Geothermal data system will need to be implemented by using shared vocabularies. A number of vocabularies have been developed by the IUGS Commission for Geoscience Information (CGI) Interoperability Working Group. These are accessible through the Geoscience Concept Definitions working group (CDTG) web page.
In particular, the lithology vocabulary developed by the CDTG should be used for lithology categories in interchange documents that have fields for lithology controlled vocabulary terms. See CGI Simple Lithology Documentation page for resources describing this vocabulary. Unfortunately, the pdf and html links from this page may cause errors in Firefox, but not MS Internet Explorer. You can right click on the pdfs and download to your desktop to view.