As with everything I investigate, I turn first to the history books. Rock candy, so popularly associated with garishly colored sticks and strings, makes its first appearance in world culinography in 9th century Iran. It seems even at such an early time in human history we had already learned to grow sugar crystals. And why would we be growing sugar crystals? Well, in many cultures these rock-like pieces of sugar are used in tea ceremonies!
Indeed, before we started stirring teaspoonfuls of granulated sugar into our porcelain cups tea ceremonies began with the depositing of a small rock of sugar at the bottom of the glass. You see, rock sugar dissolves much more slowly than granulated sugar, and as such is a perfect way to sweeten multiple cups of tea. Instead of repeatedly adding sugar to each cup, you'd simply continue to pour tea over the melting crystal stuck to the porcelain.
Now let us examine the science involved. In order to force crystal growth, we first have to create a super-saturated solution of sugar and water. Super saturation is the state of a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances. Which is to say (in this case), through the addition of heat we are able to dissolve more sugar into a small amount of water than we would be able to do at room temperature.
Don't believe that heat is a good prison for the sugar? Try this: stir three cups of sugar into room temperature water. What do you have now? A sandy, wet, messy, waste of sugar. But when the water is hot we can add sugar almost continuously until the solution is viscous and misty.
Because we have created a false prison for so much sugar (a heated solution), as the liquid cools all the dissolved sugar needs some place to go. The supersaturated solution forces sugar molecules to slam up agains each other and this is the beginning of crystallization. We also call this phase nucleation. As the molecules grow, other smaller molecules are encouraged to cling on, growing the crystal.
But, you need to give the sugar solution an easy place to start. Crystals, as I said earlier, like to cling to each other. So in order to encourage growth we first dip a skewer in water and then coat it in regular sugar. These tiny granules will act as nucleation points for the sugar as it comes out of solution, giving them an easy place to begin their geometric stacking after the skewer is submerged in the hot liquid.
Here is where things get exciting. Over the course of roughly 7 days you will be able to watch the crystals grow on the submerged skewer. They'll begin slowly, the skewer appearing merely fuzzy at first, but then they really pick up steam around the third day. Geometry reigns supreme and the crystals cluster together in cube-like structures all over the skewer and on top of each other. By the seventh day you'll be treated to quite a sight (and a delicious piece of candy). When the crystals have reached an appropriate size, remove the skewer from the solution to halt the process and let it dry hanging in an empty jar.
You can, of course, eat the candy as is. But might I suggest breaking off a lump for tea time one afternoon? Savor the slowly melting sugar over three cups of tea and relax as 4pm turns into early evening.