At 4.567 billion years old, refractory inclusions are the oldest known objects to have formed in the solar system. They are often referred to as calcium-aluminium-rich inclusions (CAI) because their chemistry is dominated by the refractory oxides of Ca and Al, as well as Ti, forming minerals such as hibonite (CaAl₁₂O₁₉), spinel (MgA_l2O₄), perovskite (CaTiO₃), melilite (Ca₂Al₂SiO₇), and a Ti-rich pyroxene called fassaite. These minerals are predicted to form in a condensing gas of solar composition leading to theories based on a hot homogeneous solar nebula where all the elements were originally in a gas state.

However, the discovery of isotopic anomalies in O, Ti, Ca, and Mg are inconsistent with a homogeneous hot solar nebula and potentially require more localised heating sources, or at least the retention of the isotopic anomalies on a localised scale. Refractory inclusions also contain isotopic anomalies attributed to the presence of short-lived radionuclides (such as ²⁶Al, ⁵³Mn, ⁶⁰Fe, ¹²⁹I) and indicate the injection of freshly synthesized material into the solar nebula. These radionuclides, particularly ²⁶Al, were potentially a source of heat for planetesimals, causing them to melt.