Making these rocks is both simple, and incredibly satisfying. As this project only requires 3 things there is minimal set up, so it's a perfect last-minute art project for older kids, or yourself if you want to make something, but don't really have the time or energy.

Every rock will look unique, and there are endless techniques to try, here I will post a few of my favorites.

As a bonus for a project that involves wax, and the melting thereof, there is surprisingly little clean up required.

This project isn't really for younger kids, as the rocks are hot. but with adult supervision this is a great craft for kids +8.

Supplies

Rocks (I like smooth round ones, but bumpy ones can give lovely patterns as well)
Crayons (any sort of crayon or colours will do)
Alto I recommend that you make sure have a white, as white helps bring out a lot of the other colours, and it marbles very well.
Paper (this is to cover your workstation, you can use Paper plates, Newspaper, scrap paper, cardboard, just make sure that its nothing that will melt)
Oven (you can also use a heat gun; I've tried with a hair dryer but while you can get it to work, it takes a while, and doesn't work as well.)
Oven mitts (to move the rocks)
Infrared thermometer (optional)

If you're afraid that your kids will burn their fingers, they can wear thin finger gloves, warning they will most likely get messy. If I'm doing this project with kids, I keep cool water close at hand, but I never really had any problem.

Step 1: Getting the Rocks

You can get rocks from almost anywhere, just be sure not to use rocks that have recently spent a while in water, like near a river or lake. as heating up these rocks can be dangerous due to trapped steam, we will not be heating them up to nearly that level but it's better to be safe.

Rocks that just get briefly wet from rain are fine. the rocks can be as big or small as you like, lighter coloured rocks work best.

Once you have the rocks you want wash them thoroughly with soap and water.

Step 2: Heating Up the Rocks

Set your oven to 350*F place the rocks on a baking tray and bake for about 6-8 minutes, ideally you want your rock to be about 170*F - 190*F, this isn't precise of you hold a crayon to it and it melts (the crayon, not the rock, if the rock melts you have an entirely new craft project) its ready.

If have a Heat gun, simply move it slowly over the Stone till it's the temp you want.

Note: the rocks cool of decently fast, you will have about 3-5 minutes of work time. So, it's best to just heat up a few rocks at a time to ensure that they remain at a workable temp.

Step 3: Prepping the Crayons and Workstation

While the rocks are baking in the oven you have time to prep the crayons, and your workstation.

If you have a box full of sad, broken, and abandoned crayons, this is the perfect time to use them. Just remove the leftover wrapper. This can be done by soaking the crayon in water, or like I did by using a box cutter to slice all the way across.

If all you have are nice, new crayons don't worry, just use the back end of the crayon, you need surprisingly little wax, and if you get to the wrapper just peel it up a bit.

As for the workstation I recommend covering the table you work on in news or scrap paper, with a paper plate for the rock, especially if you're doing this with kids, but as this craft is surprisingly tidy you can get by with just a paper plate.

As mentioned before, if you're worried keep a bowl of cold water nearby.

Step 4: Melting the Crayons

Once the rocks are hot, carefully use the oven mitt to place them on the paper plate.

Now pick your first colour, and go wild!

I like covering the whole rock in one basecoat layer, I find this helps the other colours run, and marble better.

Below are a few of my favorite techniques:

Note: I will be naming all the colours as I use them for the sake of clarity, however, please use whatever colours you have, or want to use!

Step 5: Rain Drops

Cover the whole rock in a dark blue.
Next use a light blue, hold it to the rock briefly about 1-2 seconds, you don't want to get too much wax on. repeat this all over the rock
Next with a silver crayon I placed a small dot in the middle of all of the light blue patches.
Then with White.

When the rock looked the way I wanted it, I rushed it into the freezer, this is optional, but as the colour continues to move while the rock is hot, this is a way to keep it looking the way you want.

Step 6: Layering

This technique works best with a stone that's fresh out of the oven, as you need it quite warm.

First lay down a nice thick base coat, I used a purple crayon.
Then hold your next colour on the top of the stone. hold it there till you have a sizable puddle of wax, I used white.
You can gently blow on it to help it spread out a bit.
Repeat for as long as you want. I recommend picking 3-5 colours, and repeating as if you add to many colours it can quickly turn muddy.
I used Red next
Then Silver
Then the base coat purple
Then a pinkish purple
Finally, a touch of white again.

I like to leave this kind of stone to cool of naturally, if you find that it's cooling off to fast during the layering process, feel free to hit it lightly with the heat gun, or place back in the hot oven for a couple seconds.

Step 7: Striping

This is a bit different from the others as you don't place down a base coat instead

Start from one end of the stone, colour just the end with your first colour. I used white.
Now move on to your next colour, Black for me, colour a thin line next to the white, it will grow outward so don't make it too thick.
Repeat with as many colours as you want, I just used the two.
If you want, you can leave it like this, I decided to use the black crayon to marble the lines together a bit, but as I didn't put it in the freezer you can see the lines are almost gone. that's OK I like this look as well.

Step 8: Speckling

Using a grater or box cutting knife, shave off a small pile of a few different colours of crayons.

Starting with a base coat is optional, I prefer it as I think it helps the other colours flow more smoothly. I started with a dark olive green.
Next take a few bits of chopped up crayon scatter them on top of the stone, let them run down as they want. I went for white next.
Repeat with as many colours as you want.
For me the next colour was gold
then a lighter green
Finally, a bit more gold, and white.

I recommend cooling this stone down when it looks the way you want it to, as it can change quite fast.

The stone shown in step 4 is another example of this technique.

Step 9: Have Fun!

Honestly this list could go on for a while, the possibilities are endless. You could even add things while the wax is wet, like glitter, I'm not going to as I believe glitter is evil, and I refuse to use it no matter how temptingly it sparkles at me.

Just sit down with a rock and go, you don't have to think about it, don't worry about it looking good, there is just something incredible satisfying about watching the wax melt, and swirl together.

Remember, If anyone of you make this, I would absolutely love to see what you come up with!