1. Create a new document in Photoshop (mine is 500x500 and I used a black background). Now apply a color balance layer (Layer > New Adjustment Layer > Color Balance) with the following settings:

2. These settings will make our brush a bright gold. You don't have to use those setting if you don't like that color. I just find that yellow sticks out more in renders (this step doesn't actually make a difference in the end of this tutorial, it just gives you an idea of what your brush will look like behind a render"). Now set up a brush like mine:

3. Make a few dots in your image on a new layer (under the color balance layer, but over the background:

4. Now pull out your smudge tool, and make your dots into 1 big blob. You can use whatever brush you want for smudging, but I recommend one with lots of texture:

5. Now place your smudge tool on the inside of your brushing and begin to make short strokes towards the outside of it to give it a spiky effect.

6. Select the following brush with your smudge tool:

7. Now start on the outside of your brushing and start pulling parts in to give it more shape, and variation:

8. Now we need to select that same brush as our eraser tool, and set the opacity down to 10%. Begin adding texture to your brushing by making small strokes on your brushing on the outside borders, and slowly making your way towards the center:

9. Looks good so far! You could probably use this as a brush alone if you wanted, but I will go over a few more tips to give it more texture and depth. Pick the following brush with your eraser tool:

10. Repeat the erasing process this time focusing more on the inside of the brushing. This will give your brush the last bit of texturing. After this, it is ready to be made into a brush, and used in a real abstract render!

11. Hide all of your layers except for the one containing the brushing. Invert the brushing layer (Image > adjustments > Invert) and then make it a brush (Edit > Define Brush Preset). Now you are ready to use it over and over! Good luck! Remember to experiment with this tutorial as the more creative you get, the better it will turn out!