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Is this goblet purple or brown? Well, it depends.

The cup is dichroic, meaning that it changes colour under different lighting conditions, from clear purple when light is transmitted through it, to opaque brown when light reflects off it.

Vittorio Saggiomo and colleagues at Wageningen University & Research in the Netherlands created the cup from gold nanoparticles and a 3D-printable clear plastic.

To do this, they聽mixed nanoparticles with citrate 鈥� a derivative of the acid that makes lemons sour 鈥� to create a dichroic liquid. The plastic was then dissolved in the liquid, before the solution was dried out and then 3D printed to the desired shape.

When a light is shone through聽the cup, it looks purple. This is because electrons on the surface of the gold nanoparticles absorb light of certain wavelengths, but allow violet-coloured light to pass through the plastic.

But the nanoparticles also scatter some of the light, reflecting it back at a different wavelength, so when illuminated from the聽side, the cup looks opaque and reddish-brown.

Unlike traditional dyes, the cup鈥檚 nanoparticles mean it won鈥檛聽lose its colour, says Saggiomo. 鈥淵ou can use it outside, even put it under the sun, for a hundred years,鈥� he says.

The cup draws inspiration from the 1600-year-old Lycurgus cup in the British Museum in London, made from dichroic glass that appears either green or red depending on the direction of light.

It was later discovered that the glass had gold and silver nanoparticles in it, and only six similar historical pieces survive. The Romans are thought to have created the cup by finely grinding metals and adding trace amounts to glass before it set.

While dichroic glass has been around for millennia, the team believe their method could be used to create 3D-printed filters that won鈥檛 get sun-bleached, including in optical sensors that selectively let certain kinds of light through, or for concentrating light in solar panels.

Both the plastic and 3D printing machine used are commercially available, says Saggiomo, and gold made up only 0.07 per cent of the cup鈥檚 weight, so the costs aren鈥檛 prohibitive.

Because the plastic they used is water-soluble, this particular cup isn鈥檛 practical for drinking from and shouldn鈥檛 be left outside in the rain.

The team are now experimenting with a wider range of colours and 3D-printable plastics with different properties.