This picture of Wulfenite was selected as today’s featured image on Wikipedia and I find it intriguing in the sense that the ground and background blend in so much, similar to what we see near large bodies of water, where the horizon seems to merge with the sky in the distance.
How is this blending of background and ground captured?
It's known as Infinite Coving or an Infinity Cove* - basically a constant smooth curve from floor to wall.
From How To Build An Infinity Cove For Photography
All you need to do then is light it evenly…
Click for full size
For small scale you can just use a piece of paper or card, bent into a gentle curve.
*I've always known it as infinite coving. I think the 'infinity cove' variant has come as a recent linguistic extension of such as the 'infinity pool' you get in expensive hotels ;)
Subsequent comments indicate that my preference may simply be because it was the first name I heard for it.
Possibly more US-based than my own UK, it seems this can also be referred to as a cyc [from cyclorama] wall.
When shooting small objects with a macro lens or an extension tube you can sometimes get the same effect even without the curved prop. All you need is a backdrop that matches your surface well (a white surface inside a white softbox is an obvious choice), careful lighting, and for the backdrop to be several times further away from the lens than the subject (i.e. your shiny rock should be near the front of the softbox, and the lens should be close to it — if you can't get your lens to focus close enough, it's the wrong lens.)
Because a lens focused close like this has a shallow depth of field, the backdrop and the rear part of the surface will be way out of focus, and any border between them will blur into a nice smooth gradient that doesn't attract the eye at all.
The downside of this is the usual one with macro photography: the depth of field might not be enough to cover the subject itself. A popular technique these days to deal with that is focus stacking: taking several images with slightly different focus distances and using software to combine them into a single image with the whole subject in focus. Doing that manually can be tedious, but many camera bodies have built-in support for it, so that you can click the button once and get a rapid-fire sequence of shots, each with a slightly different focus.
And it would seem that that's the technique that was used to make the image in your question: if you view the file metadata it includes the comment "Stacked from 49 images". They could have used an infinite cove, but they probably didn't have to. The 105mm focal length on a Nikon full-frame body strongly suggests that the lens used was the Nikon 105mm f/2.8G Macro.
In the US I've always known it to be called a Seamless, or a Seamless Background.
For smaller studios and independent photographers there are manufacturers of a simple metal frame that holds a long roll of heavy paper. The paper is simply unrolled to the length necessary so that the subject is comfortably within the boundry of the paper, and the paper is lit so as to remove minor shadows in the curve.
The benefit is that there is no permanent alteration of the studio space, and the portable nature of the apparatus. To change color a different color of paper is mounted.
