According to the Economist Intelligence Unit's September 2nd 2010 Country Report on Liberia, the country exported 10 million U.S. dollars worth of diamonds in 2008. This report requires a subscription to view. LIberia also exported $206.8 million in rubber and $13.3 million in gold.
You can also view the Liberian government's Financial Statistics report for March - April 2010 at http://www.cbl.org.lr/lsf_reports_details.php?record_type=External%20Trade
There are various measures of diamonds in the report. One "output" is quoted at 13,286.1 in thousands of US dollars. Search for "diamond" in the PDF to see other figures.