UV Mapping
Also called uv unwrapping, uvs, texture coordinates
UV mapping is the process of laying out a 3D surface flat so a 2D image can be wrapped onto it in the right place and at the right scale. The U and V are the coordinates of that flat layout, telling the renderer which part of the image lands where.
How it works
Think of peeling a label off a curved bottle and flattening it. UV mapping does the same in reverse: the 3D surface is cut and unfolded onto a flat sheet, and the texture image is painted onto that sheet.
Get the layout wrong and the texture stretches, tiles at the wrong scale, or shows a visible seam where the two cut edges meet. On a ring, a bad seam usually lands somewhere very obvious.
When jewelry renders need it
Plain metal does not need UVs at all, because a polished gold material is uniform and has no image to place. UVs start to matter the moment there is artwork on the surface.
That covers engraved text and monograms, printed or enamelled patterns, brushed and hammered finishes driven by a texture, and logos on the inside of a band. Anything where a specific image has to sit at a specific spot and size.
If your OBJ already has UVs, send them. If you need engraving or a pattern placed exactly, say so when you send the file and supply the artwork.
Questions about UV Mapping
Does jewelry rendering need UV mapping?
Only when a texture image has to sit in a specific place, such as engraving, a logo, or a printed pattern. Plain polished metal does not need UVs.
Can you render engraved text on a ring?
Yes, if the engraving is either modelled into the geometry or supplied as artwork with the placement you want. Send it with the CAD file.