What kind? Any ionic compound will crystallise from a concentrated solution. Copper Sulphate is a good one....Dissolve it in hot water till no more will go, then pour the solution into a cool vessel. Small crystals will form. If you take one of these out, tie it to some cotton, then reheat the rest of the solution you can use it as a seed to grow larger ones....as the solution begins to cool, lower the seed into the centre and leave it there. You should get some quite good ones.