1. <b>Investigation Methods</b>: Carbon phase assemblages (CPAs) in five ureilitic samples from asteroid 2008 TC3 were analyzed using transmission electron microscopy, Raman spectroscopy, synchrotron X-ray diffraction, and cathodoluminescence.</br> </br> 2. <b>Shock Degrees of Samples</b>: Samples MS-MU 006, MS-187, and MS-170 had low to moderate shock degrees (U-S2 and U-S3), while samples MS-MU 027 (U-S4) and MS-MU 045 (U-S5) experienced higher shock levels.</br> </br> 3. <b>Presence of Diamond Grains</b>: No diamond grains were found in samples MS-MU 006 and MS-187. Sample MS-170 contained diamond grains up to 12 μm, formed under relatively low shock pressures (5–15 GPa) with a catalytic process involving a Fe, Ni, Cr, S, P-rich melt.</br> </br> 4. <b>CPAs in Highly Shocked Samples</b>: MS-MU 027 and MS-MU 045, the highly shocked ureilites, displayed three types of CPAs: nanopolycrystalline diamond and defect-rich diamond/lonsdaleite, disordered and distorted graphite, and polycrystalline diamond with Fe-rich mineral inclusions.</br> </br> 5. <b>Formation Mechanisms</b>: CPAs with only diamond and planar defect-rich diamond (e.g., MS-MU 027) likely formed through martensitic transformation of graphite at >15 GPa and >2000 K. Assemblages of diamond, defect-rich diamond, and disordered graphite (e.g., MS-MU 045) underwent martensitic transformation followed by back-transformation to disordered graphite. No conclusive evidence supports diamond formation under high static pressure.