SRI values for marble

Has anybody had any experience documenting SRI values for marble? We have a few greenroofs that, as part of design, implement some white marble pebbles but I just havent had a clue about how to document SRI number. The American Institute of Marble recomends, according to the Natural Stone Council, that any claim of stone being a high SRI product must be substantiated through professional material testing. They also provide a few natural stones SRI measured by the American Institute of Marble but they show an average of 60, you can find this document at this link: http://www.marble-institute.com/pdfs/CaseStudy4_SolarReflectanceOfStone.pdf I know testing material through and accredited facility is the obvious option but that would increase cost and it is really difficult that and investor will be willing to do this just a few pebbles. Any idea of a source that can be used as a SRI documentation for white marble? Thank you very much.