HDRI (High Dynamic Range Image)
Also called high dynamic range image, environment map, hdr map
An HDRI is a panoramic image that stores a much wider range of brightness than a normal photo, used to light a 3D scene. Wrapped around the model, it lights the piece and gives polished metal something real to reflect.
Why a normal image will not do
A standard photo clips. Anything brighter than white is recorded as white, so the difference between a lamp and a sheet of paper is lost. An HDRI keeps that difference, storing brightness values far above what a screen can display.
That range is what a renderer needs. When an HDRI of a photo studio is wrapped around a model, the softboxes in it light the scene at their real intensity and cast the same highlights a physical studio would.
What it does for metal and stones
Polished gold is a mirror, so its whole appearance is a reflection of the surroundings. Light it with a bare lamp in an empty scene and it reflects nothing, which reads as dull grey. Light it with an HDRI and it reflects a room, which reads as gold.
Stones need the same thing. The bright and dark bands you see across a brilliant cut are reflections of the environment, and swapping the HDRI changes how a diamond sparkles as much as changing the stone would.
For ecommerce work the environment is chosen to give clean highlights while the visible background stays white, so the piece is lit like a studio shot without a studio showing up behind it.
Questions about HDRI
Do I need to supply an HDRI?
No. Lighting setup is part of the render. You send the CAD file and the material details, we handle the environment.
Does the HDRI show up in the background?
Not in your delivery. Renders come back on a white background. The HDRI is doing the lighting and supplying the reflections, not appearing as scenery.