Tone mapping
The process of converting a rendered image's high dynamic range into the limited range a display can show.
Why renders need it
A rendered scene, like the real world, contains bright light sources and deep shadows. Screens cannot show that whole range. Without tone mapping, every bright area clips to flat white and colours shift as they get brighter.
Common curves
Renderers offer several curves. Filmic curves such as ACES compress highlights smoothly, the way film does, and stop bright areas from clipping harshly. Simpler curves such as Reinhard are gentle but can look flat. Newer curves aim to keep colours more faithful as they brighten.
What it means for jewelry
Jewelry images are full of tiny, very bright highlights on metal and stones. Good tone mapping keeps those highlights bright without turning them into white blobs, and keeps gold from drifting towards orange or green as it brightens. Rose gold and yellow gold are especially sensitive.
Exposure works together with tone mapping. If a diamond looks grey, the answer is usually a better gem environment rather than more exposure, because raising exposure also flattens contrast in the metal.
Checking your output
Judge colour on the exported image, not only in the viewport, and check it on a phone as well as a desktop monitor. Keep the same settings across a collection so that variants are comparable.