Based on the dimensions you provided and the photograph, the density is likely between 5 and 9 g/cm3, assuming cylindrical volume with height between 2.8-3 inches and radius between 0.4-0.5 inches. Based on this density and the weak magnetic properties, you have an iron-rich rock. Most fossils are comprised of calcite or a mineral with a similar density of around 3 g/cm3, with the exception of cases where metallic minerals replace the original material.
The location provided is useful. However, I could not find a Cressy River, and I assume you are referring to the Merced River. This area flanks mountains that are rich in gold placer deposits and associated minerals, including iron. It is most probable that you found a iron-bearing mineral associated with the igneous or metamorphic rocks in the mountains.
GEOLOGIC MAP OF THE SAN FRANCISCO - SAN JOSE QUADRANGLE
Densities of common minerals
Densities of Metals