Just to avoid any confusion up front, you cannot plant a seed crystal in the ground and grow a lovely crystal tree. It’d make for great reading, but I’m afraid it’s quite unrealistic. A seed crystal is used to grow larger crystals, but only when “planted” in a supersaturated solution of whatever compound you hope to make a crystal from. The seed crystal serves as a central core, and growth occurs on its surfaces. The best crystals are grown slowly, on very regular seeds. “Regular”, in this case, means that the crystal is a single structure and has well defined faces. An “irregular” seed could be several crystals that fused together, having a more jagged appearance. Irregular seed crystals still grow, but the final crystal will appear to be several overlapping structures and may include cavities (holes). This can be visually interesting, as with copper (II) sulfate, but for the purist who desires a single symmetrical crystal, obtaining a good seed crystal is imperative.
Growing a seed crystal isn’t hard. The process is very similar to crystal growing in general. The biggest difference is that if you aren’t too concerned with size and quality, you can rush it. To start a crystal, you’ll need to make a supersaturated solution of your compound. This is usually accomplished by dissolving a quantity of the compound in water (or alternative solvent) at an elevated temperature. If you have a solubility table (and know how to use it), then you can quickly choose a temperature and amount of chemical that will produce a supersaturated solution when cooled. For those with less literature and/or chemistry expertise, have no fear. You can easily prepare a supersaturated solution as well. Start off with water at room temperature, and stir in as much of your crystalline compound as will dissolve. (You have a saturated solution at this point.) Now heat your mixture and stir in more of the chemical. Make sure that you don’t add so much that some sits as a solid on the bottom; you want it all to dissolve. If you do add too much, then add a little more water (slowly, so you don’t dilute it too much) and keep stirring until everything dissolves. When you let the mixture cool back down, it will be a supersaturated solution, just like you always wanted.
You have several options now for how to grow those seed crystals. You’ll have to make your choice based on time and quality.
The quickest way to get some very small crystals (but generally not of very good quality) is to pour a portion of your (still hot/warm) mixture into a shallow dish and let it evaporate. The sides will be coated with small crystals, left behind. Sometimes you’d be just as well off using the chemical straight from the bottle as a seed, but these small crystals can be used as seeds. If you were just looking for a few to roll in a string to make a chain of crystals, you’re in business. For one big, beautiful crystal you’ll need another method.
It takes longer (a couple days), but if you simply let your solution sit to cool, covered, and undisturbed, crystals will slowly form. The longer you let it sit (within limits) the larger and better seeds you can get. You can simply pour it all out and choose the crystal you like best to work with when you are ready. Remember – look for one that has the best shape, not necessarily the largest one. There’s no real limit to how big your final crystal can be grown (though there are practical limits), and the starting size of the seed crystal isn’t going to make a big impact on the final result.
If you really have lots of time to work with, you can carry out a similar procedure to the one above, only you control the rate of cooling. For instance, you can put the container in a water bath that will hold the temperature constant. Then you slowly (the slower the better – days, weeks…) lower the temperature to encourage the formation of a single crystal to work with. You should be able to get the very best seeds this way, but it will take a lot of patience. You should also make sure that whatever you’re growing doesn’t encourage anything to grow…algae, bacteria, etc. Keeping it in a dark place will help if such is the case. You might not expect such things to happen, but I’ve seen it, and it was rather disgusting.
Good luck to you, crystal grower. Don’t underestimate the value of a good starting seed. My dad put in the time to do it right with me when I was small, and I still have and treasure the exquisite nested alum crystal we grew those twenty-odd years ago.