I was wondering if a core biopsy was done rather than a fine needle aspiration because it's solid?

No. Fine needle aspiration (fna) biopsies can be performed on either solid or cystic masses. An fna yields cells/cellular clusters, while a tissue core provides a glimpse of how the cells interact with surrounding tissue. In certain situations, observation of that interaction provides the pathologist with enough information to specifically diagnose a tumor versus a more general diagnosis (with fna).
FNA is informative. Core biopsy can show histology of the lesion and invasion if we are talking about tumor. Fna is perfect for solid and cystic lesions too, but sometimes not easy to document invasion.
Either possible. Fine needle aspiration(fna) uses a 21-18g needle to pull cellular material from a target lesion for cytologic evaluation. Core biopsy uses 14-9t needle/device to obtain tissue samples/pieces for histologic evaluation. This provides additional architecture information.