Next up was the base. Looking at pictures, we realized the color of the stone was dependent on lighting. Nathaniel selected Tamiya buff (XF-57) as a good place to start.
XF-57 Buff Tamiya opaco
Tamiya 81772 Acrylic Mini XF72 Brown JGSDF 1/3 oz - Walmart.com
XF-57 Buff 10ml Tamiya
Amazon.com: Tamiya America, Inc acrílico XF57, Flat Buff, TAM81357 : Arte y Manualidades
Tamiya 81768 - XF-68 - NATO Brown - 10ml Acrylic - Hub Hobby