Jewelry 3D models are dark mainly due to insufficient lighting setups or incorrect material properties (like low reflectivity) in 3D software.
Jewelry materials such as metal or gemstones rely on light reflection and refraction to look bright. Without enough key/fill lights or accurate material settings, the model can’t catch light properly.
To fix it: Add soft fill lights or use an HDRI map for even illumination. For materials, ensure high metal reflectivity and enable gemstone refraction.