A special kind of lab-created opal can be used to carry several encrypted messages at the same time. Find out how it can keep secrets for someone — and tattle on that same person — depending on what you spray it with.
To do so, this lab-grown opal is applied to a metal chip in a very precise way. Crystals are then grown on the chip from a point where they create a sheen of, well, opalescence, that causes the chip to shimmer in the light. The gleam is even over all areas of the chip, until a certain concentration of water and ethanol is applied to the surface. This doesn't damage the chip in any way, but the water bead and flattens in a recognizable pattern; words. The opal on the chip is sometimes deeply porous in ways that aren't noticeable to the naked eye. Water and ethanol molecules, though, can distinguish easily between one kind of surface and another, and react accordingly. A spy could theoretically be given a chip that — when 5% ethanol solution is applied to the surface — says, "The raid must be scheduled for midnight tomorrow."
Once that spy has smuggled the chip across a border and given it to his superiors, they can apply a 10% ethanol solution to the chip only to have new words appear: "Don't trust this guy. He's a double agent. Schedule the raid for ten o'clock tonight." Different concentrations of ethanol can cling in different ways, making the surface able to hold several different messages at once. Once the material dries, it again looks blank and spotless.
This material, called Watermark Ink, is being perfected by materials scientists at Harvard University. In all likelihood, it probably won't be used extensively in spycraft. The coded messages will likely say things like, "This isn't unleaded gas," or "You've just spilled nitroglycerin, you fool!"
A heavily encoded chip covered in watermark ink could be dunked in mysterious liquids spilled in a lab or along the road in a traffic accident. This could allow hazmat employees to figure out what chemical they're dealing with. One exposure and they can literally read the name and composition of the material off the chip, then wipe the chip dry and reuse it. It could also be used by everyday people to figure out if they're getting ripped off when it comes to buying special gasoline.
Then again, if ordinary people carry these chips all the time, those opal chips carried by spies will be much less suspicious. A shimmering chip wouldn't even raise an eyebrow at a border if everyone had one. Oh, these intelligence people are crafty.