Final answer:
The presence of dark gray and white wavy bands in the rock, indicating a foliated texture and alignment of minerals, supports Jenny's conclusion that the rock is metamorphic, specifically gneiss, which is a result of high-grade metamorphism.
Step-by-step explanation:
The observation that best supports Jenny's conclusion that the rock she found is metamorphic is the presence of dark gray and white wavy bands. This banding is indicative of a foliated texture, which is a key characteristic of metamorphic rocks like gneiss. Foliated textures are formed under the influence of directed pressure and high temperatures, which align minerals in the rock to give it a layered or banded appearance. Considering the minerals present, the probable parent rock of gneiss is often granite or schists. The degree of foliation and the presence of minerals like feldspar and quartz suggest that the rock underwent high-grade metamorphism.
Based on the information provided, Jenny can compare her observations with a Rock and Mineral Guide to see the similarities with gneiss, as the texture, mineral content, and the appearance of bands align with the description of gneiss in such resources.