Let me start out with how to grow crystals.

You need an absolutely clean container.  Baby food jars will work, but they are 
awfully small for growing decent sized crystals.  Smallish glass baking dishes, 
say 5"x5" are much better.  I use a 250 ml or 500 ml beaker, covered with 
parafilm (or Saran wrap), so something about the same size (like a Mason or 
fruit jar) is ideal.

You need a place to grow the crystals that is not disturbed by pets, younger 
children, or parents.  This place should have as constant a temperature as 
possible - meaning that window sills are pretty much out. I use the top of a 
cabinet or closet shelf, or sometimes the refrigerator (in the lab, of 
course...). You will need a small plastic spoon, one small enough to reach into 
whatever kind of container you are planning to use to grow the crystals. And 
finally, you will need a couple of 1-liter bottles to hold the solutions.  I use 
plastic soda bottles, *BUT* unless you have a child-free place to keep them, I'd 
use something else.  A litre of even a non-toxic chemical can make a young child 
quite sick.

The first order of business is to understand *CLEAN*.  Everything you use to 
grow crystals has to be CLEAN, CLEAN, CLEAN.  That means, no lint, no 
fingerprints, no water spots, just sparkly spotless squeaky clean!. The best 
home technique is to wash the containers with liquid detergent and a brush, then 
rinse them out with hot water until the soap is entirely gone, then rinse them 
twice with a small amount of distilled water and turn them upside down on a 
paper towel to dry. Be sure to clean the tops the same way.

Make a saturated stock solution in the following manner:  Put 800 ml of hot (not 
boiling, about 120 to 140 degrees) distilled water in one of the 1 litre 
bottles, and slowly add the crystal chemical to it with stirring or shaking. 
When no more of the chemical will dissolve, leave the bottle undisturbed for one 
hour, then pour off the solution from the first bottle into the second bottle. 
Try hard not to get any of the undissolved chemical into the second bottle. 
Then, add 50 ml of additional distilled water to the bottle. This is your 
crystal growing stock solution.

Make doubly sure that your crystal growing container is *CLEAN*.  Then, pour in 
the (still warm) stock solution to a depth of 2-3 cm.  Cover the container 
*loosely* with aluminum foil, and set it aside to start to crystallize as the 
solution cools. You may want to check it two or three times each day. Do not 
stir, shake, bump, or get dust in the container. Usually after 2-3 days there 
will be small crystals forming in the bottom of the container. Let them grow in 
the original container until some of them are 5-10 mm long.  You may have to 
_SLOWLY_ add some additional stock solution to the container. For the best 
results and the biggest crystals, try to disturb the crystals as little as 
possible. Put them on a back shelf, hang out a DO NOT DISTURB sign, and leave 
them alone for several days.

Once you get some nice small crystals, pour the solution in the crystal growing 
container back into the _first_ bottle, and dump the crystals out onto a paper 
towel.  Select a few of the nicest looking crystals as seeds to start the second 
part of the process, and put the rest back into the first bottle.  Now clean, 
and I mean -CLEAN- out the crystal growing dish (or jar), rinse it twice with 
distilled water, and fill it with stock solution to a depth of 2-3 cm (just like 
before).  Now, use the plastic spoon to gently lower just one of the seed 
crystals into the solution.  Cover the container *loosely* with aluminum foil or 
Saran wrap and set it aside for the crystal to grow.  I usually start five or 
six containers on this second phase.

Now, once per day, uncover the growing container and use the plastic spoon to 
_gently_ turn over the growing crystal.  Replenish the level of stock solution 
in the growing jar, but take care not to pour new solution in so quickly that 
you disturb the growing crystal.  Keep this up as long as the crystal continues 
to grow true.  If additional crystals start to form, then use the spoon to fish 
out the big crystal, and dump the solution and the little crystals back into the 
first bottle, clean the dish, and start over with the big crystal as a seed.

You can go through a lot of stock solution this way.  If you run out, then just 
fill up the first bottle (with the crystals in it) to 800 ml with distilled 
water, dissolve more new chemical mixture, and do like before to make new stock 
solution.

There are two keys to growing big, beautiful crystals this way.  The first key 
is CLEANLINESS.  Dust is your enemy, big time.  The second key is PATIENCE, 
crystals grow as fast as they want to, not as fast as _you_ want them to grow.  
This is not a short-term, overnight or even over-the-weekend project!  Still, I 
have grown crystals 3-4 cm long in 4 weeks, and I have grown some 10 cm crystals 
(over a pound!) in six months. When the crystal is as big as you want it to be, 
take it out and dry it. If you want to, you can spray it with clear lacquer to 
keep it permanently.

Now, chemicals.  I put this off to last, because what you use depends a lot on 
how much you can be trusted with chemicals or the likelihood of little kids 
messing with your crystal factory and makin's... I have listed some 
recommendations below, along with how toxic the stuff is. 

All of these crystals are sufficiently non-toxic that the dried crystals can be 
handled with bare hands. Needless to say, you shouldn't eat them!

These are my recommendations for chemicals to use to grow crystals:

1.  Aluminum Potassium Sulfate (Alum) - non-toxic, cheap.  Grows clear,
    colorless octahedra.  Fairly fast growing, easy to succeed.

2.  Aluminum Potassium Sulfate + 25% Chromium Potassium Sulfate.  non-toxic,
    inexpensive.  Grows beautiful purple crystals.  Fast growing, easy to
    accomplish good results.  My second favorite combination.  You can
    grow chromium potassium sulfate by itself, but the crystals are so
    dark you can't see through them.

3.  Sodium sulfate.  non-toxic, cheap.  Grows clear triclinic blades.
    non-toxic, fast growing.  Medium-difficult to get well shaped crystals.

4.  Ferric Ammonium Sulfate.  non-toxic, inexpensive.  Grows pale
    lavender crystals, nearly always twinned.  Slow growing, Medium-easy
    to get good results.

5.  Copper Sulfate - toxic if ingested.  Grows bright blue triclinic
    crystals.  Fast growing.  Medium-easy to get good results.  Except for
    not being non-toxic, probably my first recommendation.

6.  Potassium ferricyanide.  toxic if ingested.  Grows bright orange
    isometric crystals.  Slow growing, easy to get good results if you
    have enough time.

7.  Copper (II) Acetate.  toxic if ingested.  Grows deep forest green
    rhombic crystals.  Slow growing, difficult to get well formed crystals.
    But the results can be really beautiful, and worth the trouble.

8.  Sodium chloride (table salt).  non-toxic.  Grows clear cubes.  Slow
    growing, difficult to obtain well-formed crystals.  Very sensitive
    to changes in temperature or humidity.  Transparent cubes are
    obtainable, but a LOT of effort and care is required.

9.  Potassium chromate (not dichromate).  You will probably have to make
    this by reducing Potassium Dichromate with Potassium hydroxide.
    Dissolve the dichromate, and then add potassium hydroxide slowly
    until the color _just_ changes to bright yellow.  Toxic if ingested,
    the dichromate is listed as a carcinogen (also if ingested). Grows
    bright lemon yellow monoclinic crystals, just beautiful.  Medium
    fast growing, medium easy to grow good crystals. Gorgeous.

10. Lactose.  non-toxic (edible!).  Medium-slow growing, easy to get good
    looking clear to white crystals.  Changing containers often is necessary
    because the lactose tends to want to grow mold.

11. Ferrous Sulfate (coppereas).  Mostly non-toxic, grows pale green rhombic
    crystals.  Fast growing, cheap.  Medium difficult due to air oxidation
    of solution - requires really good cleanliness.  The crystals can be
    transparent and grow to large size.

12. Cobalt Chloride.  toxic if ingested.  Fairly expensive, medium slow
    growth.  Crystals are deep red.  Medium difficult to get good results.


Crystal gardens are nice, too.  I use a mixture of ammonium and sodium sulfate 
because it grows really nice long blades and effloresces if the humidity is not 
too high.  You can color the gardens with soluble prussian blue (Mrs. Somebody's 
blueing) if you want to. You know this trick, right? It's the one where you pour 
the salt solution over charcoal briquettes in a pie plate.  You can do the same 
thing if you make an absorbant tree out of paper (use a paper clip to stiffen 
it, it gets soft when it gets soggy) and add green food color to the solution.

Here's an alternate method that gave somewhat quicker results.  Choose a 
chemical that is soluble in alcohol, but less soluble in water. Dissolve it in 
alcohol to make a saturated solution like above, put it in a small jar covered 
with a osmotic membrane (you can use filter paper, even) then sink it in a 
larger jar of water. As the water diffuses in and the alcohol diffuses out, the 
chemical crystallizes...  As I recall, copper sulfate was used, but this was 
years ago.

I think Alum is the cheapest chemical, and you can get it in grocery stores for 
39 cents/pound - add this to your chemical list.  It's sometimes sold under the 
name "pickling salt".