Yes, scientists have found evidence of stars that are essentially giant diamonds.
These celestial objects are white dwarf stars that have crystallized. The video reference mentions a star nicknamed "Lucy" (after The Beatles song "Lucy in the Sky with Diamonds") whose core is primarily carbon. As the star cools over billions of years, the carbon atoms compress and crystallize, forming a massive diamond structure. Approximately 90% of Lucy's mass had crystallized.
Therefore, "diamond star" is a descriptive, albeit informal, term for crystallized white dwarf stars composed primarily of carbon.
