Pack

one of them answers the light differently.

tear the foil, thumb the stack, and move the light across each card -- the rare one gives back a colour instead of a shine

A metallic ink is a mirror: tilt it and a white highlight slides across the surface, but the colour of that highlight never changes. A holographic card is not a mirror at all, it is a diffraction grating -- microscopic ridges that send a different wavelength back to your eye at every angle, so tilting it changes the hue rather than the brightness. That is the whole difference between a shiny card and a rare one, and it is why anyone who has opened a pack can tell them apart across a room without reading a word. The same trick works on a banknote strip, the underside of a CD, and a beetle shell. The cards are real minerals, and each one carries its lustre -- metallic, vitreous, adamantine, pearly -- which is the same question asked of a stone in the hand: how does it answer light? Opal is the secret card because opal is the one mineral that does what the holo does. It is silica spheres packed in a lattice, the lattice diffracts light, and so the colour of an opal changes as you turn it. A grating that grew.

Concepts: diffraction, reflection, light, rarity.

An interactive you play with one thumb, on Wundr.