1 cup water
2.5 cups sugar
2 feet string
Black or white craft foam
Tie one end of the string to the pencil. The string should be just long enough that a couple inches touch the bottom of the jar. Dampen the string and put it in the sugar. Keep the bottom couple of inches dry and free of sugar. The sugar crystals that attach to the string will help seed the crystals. The crystals will grow around these starter crystals. Set the string aside to dry.
Heat the water to a boil. Stir in the sugar until it is dissolved. Let it cool. Pour the sugar water into the glass jar and put the string into the jar. Set in a cool place. You might want to cover the jar with plastic wrap to keep bugs, etc. out.
You can add food coloring to the solution to make the crystals pretty colors. Thanks to Pat Hess for the suggestions.
In a couple days, you should start to see crystals forming on the string and on the sides of the glass jar. In a week or so you should have a nice size crystal. Don't let it grow too much or crystals will form on the bottom and make it difficult to remove the string.
Take the string out of the water and hang it up to dry. Examine the crystal under a magnifying glass. What shape are the crystals?
To make it into a neckerchief slide, cut out a piece of black or white craft foam. Punch or cut two holes in the foam. Remove the crystals from the bottom couple inches of the string. Put the ends of the string in the holes. Either tie the ends together or simply hot glue the ends to the back of the foam. Hot glue a PVC ring to the back.