Neutronium, which refers to the hypothetical material found in the core of a neutron star, is an extremely dense and exotic substance. It consists primarily of tightly packed neutrons, with only a small fraction of protons and electrons present.
Given its extreme conditions and the absence of light, neutronium itself would not emit or reflect visible light, so it would appear black in the traditional sense. However, it's important to note that neutron stars are surrounded by extremely hot and bright environments, such as accretion disks or neutron star atmospheres, which can emit intense radiation across various wavelengths.
As for its texture, smoothness, and hardness, it's challenging to make definitive statements since neutronium exists under extreme gravitational forces and pressures. Its characteristics are not easily comparable to familiar materials found on Earth. Neutronium is thought to be incredibly dense and have a solid or superfluid-like state. It would likely have a smooth and uniform appearance on a macroscopic scale, but its microscopic structure is not well understood due to the complexity of the neutron star's interior.
It's important to mention that neutronium is a theoretical concept, and our current understanding of its properties is based on models and calculations. Direct observations or experimental studies of neutronium are not possible at this time, so our knowledge about its exact appearance and properties is limited and subject to ongoing scientific research and theoretical exploration.