After a little research on Instructables.com, I didn't find an instructable which show you how to grow big and beautiful crystals. So I think that this instructable is a good idea.
Growing crystals is more than only a instructable, it is for me a passion, so a little piece of my soul was caught in them (in crystals).
For me, the word "crystal" means something like a DNA, because natural crystals have been created long time ago, and they have a piece of time history in them.
In this instructable you'll find how to grow several crystals and tips and tricks for growing them.
To grow a big wonderful crystal or a beautiful one, you may read all this instructable.

Step 1: A Little Introduction in the World of Crystals

What is a crystal ?
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an ordered pattern extending in all three spatial dimensions. In addition to their microscopic structure, large crystals are usually identifiable by their macroscopic geometrical shape, consisting of flat faces with specific, characteristic orientations.(Wikipedia)

In this big world, are a lot of crystals and these crystals also have a lot of applications.
For example, your computer's processor is almost made from silicon, which is a crystal.
Or your keys, that are almost made from iron, which is a crystal too.
Or your wedding ring (if you're married), which I think it's made from gold, another crystal.

But crystals aren't all the same, they have a different structure and different properties.

Enough with talking. Let's start the work!!

Step 2: How to Make the Solution (slow Evaporation Method)

Needed materials:
- 100 g or more crystal powder (If you want to grow a big crystals, you should have more than 100 g)
- 100 ml distilled water (or boiled water)
- patience
- filter paper
- 2 clean jars

1. Mix the crystal powder and the water.
2. Stir until the powder stops dissolving.
3. Filter the obtained solution. Don't worry if the powder wasn't all dissolved. You should keep the powder remained on filter paper.
4. Let the solution for a while. A big crystal isn't growing a sudden.
5. Next time you'll take a look at your crystals, you'll see that there have grown many crystals there. Choose one that you like (a seed) and then put the remained crystals where you put the remained powder.
6. Filter again the solution and then put your little crystal in solution. (You may do that every time you'll see little crystals.)
7. When you see that in the jar isn't enough solution to cover the whole crystal, make some new solution as you have made the first one (I mean make the solution in a clean jar, filter it and then fill the jar that have you're crystal).
8. Growing crystals takes a lot of time, so be patient.

Tips(applied for all crystals):

You should cover your jar with something, maybe a filterpaper, because the solution can get some impurities from the air and then will grow many "parasitic" crystals
If you want to quickly grow many little crystals, you can heat the water, but if you want to grow big nice crystals, you shouldn't do that.
Don't put your jar on a heat source.
Be careful not to break the crystal. (If you break it, you'll be really dissapointed, trust me.)
Don't taste the crystals.
If you have some small crystals sticked of your crystal, put your crystal in water and let it there until you don't see the small crystals.
If you want to quickly grow a little crystal, you can put a little "crumb" from powder as seed.
WARNING: DO NOT TOUCH OR TASTE THE CRYSTALS OR THE SUBSTANCES USE FOR MAKING THEM!! SOME OF THEM ARE HIGHLY TOXIC (POTASSIUM DICHROMATE, POTASSIUM NITRATE or IODINE COMPOUNDS)!!! IF YOU WANT TO TOUCH OR TASTE THEM ASK A SPECIALIST!!

Step 3: Growing the Copper Sulphate Crystal

Needed materials for this crystal:
- 100 g Copper sulphate(CuSO₄) and the other materials (water, filter paper ...)

These crystals are three months.

Warning!!: The copper sulphate is toxic, so don't taste it!!

Step 4: Growing the Aluminum Alum Crystal

Needed materials for this crystal:
- 100 g Alum(KAl(SO₄)₂) and the other materials (water, filter paper ...)

This crystal is three months.

This crystal isn't toxic, so you can taste it.
In the past, maybe now too, it was used to stop the bleeding.

Step 5: Growing the Potassium Dichromate Geode

Needed materials for this crystal:

- 200 g Potassium Dichromate(K₂Cr₂O₇) and the other materials (water, filter paper ...)

For this "crystal", you can keep almost all the crystals deposited on the bottom of the jar.

After several days, you can hold the geodes in your hand.

These geodes are growing quickly and they needn't so much care as the other crystals need.

Warning!!: The Potassium Dichromate is highly toxic, so don't TASTE it or TOUCH it without gloves!! DO NOT LET A KID PLAY WITH IT! DEPOSIT IT IN A WELL-VENTILATED AREA!

Step 6: Growing Nickel Sulphate Crystals

Needed materials for this crystal:
- 100 g Nickel Sulphate (NiSO₄) and the other materials (water, filter paper ...)
Here you can obtain two types of nickel sulphate crystal, How you can do it ? It's simple.
This crystals have a "special" property, if when you heat the solution up to 30° C, you get turquoise (turquoise is the color) crystals, and if the solution's temperature exceeds 30° C, you get translucent green crystals.

If you want, you can prepare the solution, and then you can add a little crystal as seed.

Warning!!: The nickel sulphate is toxic, so don't TASTE it or TOUCH it !!

Step 7: Growing Rochelle Salt Crystals

Needed materials for this crystal:
- 130 g Rochelle salt(Potassium sodium tartrate KNaC₄H₄O₆) and the other materials (water, filter paper ...)
Tips: These crystals should be kept in a constant temperature environment.

These crystals have a special property, they are piezoelectric.
They produce electricity when you break one, when you yell at it ...
Don't think that they produce a lot of electricity!! You'll be dissapointed when you'll see how much electricity it produces.

Step 8: Growing Chrome Alum Crystals - Obtaining the Crystal's Solution and Growing the Crystal

How to obtain the solution:
I weighed 14.7 g K₂Cr₂O₇ (0.05 moles) and mix it with 12.5 ml of 4 M H₂SO₄ concentration (0.05 mol) and 100 ml of distilled water. We warmed slightly to obtain the solution. According to reaction's stoichiometry, we need 0.15 moles of SO₂ can be obtained from 0.3 moles of Na₂S₂O₃ (52 g) and 0.15 moles of H₂SO₄ (about 40ml).
Sodium thiosulphate was introduced in sulfur dioxide generator consists of an Erlenmeyer flask which was attached to a plug with a hole to put a separating funnel to add sulfuric acid in small portions. Heat gently to ensure a constant flow of gas.
For bubbling I used a rubber hose attached to a tube inserted in the second hole of the stopper. During the reaction solution I took care that the temperature does not exceed 40 ° C to avoid side reactions.

Tips: For cooling the solution during the bubbling, I used an ice bath.
During making the solution, don't exceed 60 °C because exceeding this temperature create complex chrome combinations.

Growing the crystals:
After obtaining the blue-green solution, you let it for a while. When you see many crystals grown on the bottom of the jar, choose one or two, and then dissolve the other.
You will obtain dark purple crystals.

This way to obtain Chrome Alum is thought by me because we haven't had Sodium metabisulfite as it was written in the originally recipe .

Warning!!: The chrome alum is toxic, so don't taste it!!

Here are the reactions:

K₂Cr₂O₇+ H₂SO₄+3SO₂ →K₂SO₄+Cr₂(SO₄)₃+H₂O
2Na₂S₂O₃+H₂SO₄ →SO₂+2S + 2Na₂SO₄ (sulfur dioxide generator)
K₂SO₄+Cr₂(SO₄)₃+ 12H₂O → 2KCr(SO₄)₂·12H₂O

Step 9: Mixing the Aluminum Alum and Chromium Alum Solution to Obtain a New Crystal

Because both Aluminum and Chromium have almost the same formula and structure, you can mix them together and get a new nice crystal.
For obtaining the new crystal, you have two ways:

Mix a part of Aluminum Alum solution with a part Chromium solution. You'll get light purple crystals.
Mix the two solution together and then add a Aluminum Alum crystal as seed. You'll get a two colors crystal - inside, it is clear, and outside, purple.

Step 10: Obtainin a "Gold Rain"

Procedure: In a beaker mix 25 ml 0.2 M solution of Pb(NO₃)₂ and 0.25 ml of KI with 0.4M concentration. Observe the formation of an abundant amorphous precipitate of lead iodide. Then let it decant and add hot distilled water in a ratio of 1:4 to the yellow suspension initially obtained, then heat to dissolve the precipitate. If it doesn't completely dissolve, distilled water is added and continue heating. By slow cooling, we observe the formation of small "flaps" of gold that are actually lead iodide. By rapid cooling, is observed a large number of crystallization centers that give the appearance of yellow-pearl.
The reaction is:
Pb(NO₃) ₂ +2KI → PbI₂ + 2KNO₃.

WARNING: DO NOT TASTE OR TOUCH THE SUBSTANCES!!

Step 11: Obtain "Stars" in Gelatin

To obtain single crystals of magnesium hydroxide I used slow diffusion of reactants technique in a gel. For this purpose I mix 8g gelatin gel with 200 ml of distilled water to prepare the gel environment. I added 2 ml of 10% solution of MgCl₂ and I shake it for homogenization. I introduced the gelatine in a graduated cylinder and waited to strengthen. Meanwhile I prepared 5 ml of concentrated ammonia solution 5M that I added over gelatin. Crystallization process is very slow. In the early days there were few white rings of precipitate formed (Liesegang's rings). After two weeks have been shaped crystals of magnesium hydroxide.
The reaction:
MgCl₂+ 2NH₄OH → Mg(OH)₂ + 2NH₄Cl

Step 12: What Can You Do With Them ??

I'm thinking about a lot of applications.
When you're sure that you don't want to grow them bigger, just spray them with some varnish.
One of them is that you can make handmade jewelry like earrings, rings or pendants. (ONLY WITH NONTOXIC CRYSTALS)
You can use them to show your friends how great is chemistry.
Stay tuned. I will show you how to make some jewelries with them.

Step 13: And That's It!

I hope you liked my crystals.
By the way, sorry for my bad english.
And a little video for you. You can see how the silver is growing on the copper wire. Enjoy!

Step 14: For Make-to-Learn Youth Contest

What did I make ?

I grow some crystal to show people how great are them and how great is chemistry. First time I was in the chemistry lab I wanted to grow some crystals. They aren't just nice, they can be useful too.

How did I make it ?

I just got what I was needing, I made the solution, let it dry, and then grow more crystals. I know that it looks easy to grow, but it isn't.

Where did you make it ?

I have made them at school, in the chemistry lab, but you can also DIY them at home.

What did I learn ?

I learn that if you put a piece from your soul in your projects, it can become something great. To make something nice, you must like it, you should work hard and you may be patient. Great thing haven't appeared in one night. I saw a lot of great things made using crystals.