Mineral fillers can be added to thermoplastics commonly used in fused deposition modeling (FDM) 3D printing to enhance their performance. Acrylonitrile-butadiene-styrene (ABS) and polylactic acid (PLA) composites were prepared with powdered minerals by co-extrusion to produce filaments for FDM. The minerals were surface modified in some filaments to determine how interfacial bonding between the polymer and inorganic fillers affects the strength of the composite. Composite filaments containing 1-5% by weight silica or basalt were examined. Specimens were 3D printed using a commercial Flashforge Creator Pro FDM printer for tensile testing. The structure of the composites was also examined by scanning electron microscopy (SEM) to determine the effect loading of inorganic fillers has on the morphology of the composite thermoplastics. This work provides a method of incorporating earth abundant minerals into the manufacturing of composite materials.