The advent of technologies such as magnetic resonance imaging (MRI)-guided radiation therapy has led to the need for phantom materials that are capable of producing tissue-like contrast on both MRI and computed tomography (CT) imaging modalities. The purpose of this work is to develop a system of easily made and formed materials with adjustable T1 and T2 relaxation times, and x-ray attenuation properties, for mimicking soft tissue and bone with both MRI and CT imaging modalities. The effects on T1/T2 relaxation times and CT numbers were quantified for a range of Gd contrast (0-25 μmol/g), agarose (0-8% w/w), glass microspheres (GMs) (0-10% w/w ) and CaCO3 (0-50% w/w) concentrations in a carrageenan-based gel. 105 gel samples were prepared with the additives, carrageenan and water. Samples were imaged in a 3D-printed holding structure to find the attainable range of T1/T2 relaxation time and CT number combinations. T1 and T2 relaxation time maps were generated using voxel-wise inversion-recovery and spin-echo techniques, respectively. A multivariate linear regression model was generated to allow the materials system to be generalized to semi-arbitrary T1/T2 relaxation times and CT numbers. Nine diverse tissue types were mimicked for fit model validation. The achievable T1/T2 relaxation times and CT numbers for the additive concentrations tested in this study spanned from 82 to 2180 ms, 12 to 475 ms, and -117 to +914 Hounsfield Units (HU) respectively. The mean absolute error between the fit model predicted and measured T1/T2 relaxation times and CT numbers for the nine tested tissue types was 113±64 ms, 16±26 ms and 11±14 HU respectively. In conclusion, we have created a system of materials capable of producing tissue-like contrast for 3.0T MRI and CT imaging modalities.© 2020 Institute of Physics and Engineering in Medicine.