Yes, plain old table salt works. But for the most satisfying, fast-growing, and visually stunning results, (found in the spice aisle) or borax (found in the laundry aisle) are your best friends.
If you want a specific shape, bend a pipe cleaner into that form (like a star or a heart). If you want a natural cluster, simply tie a piece of rough twine to your pencil. growing crystals
While most household crystal projects are safe, always supervise children. Borax should not be ingested, and sugar solutions get incredibly hot. Always use heat-resistant glass like Pyrex to avoid jars cracking from the boiling water. Yes, plain old table salt works
Add a few drops of dye to your water to create "rubies," "sapphires," or "emeralds." and visually stunning results