Such a pretty look.  These rely on the dirty pour technique, that means blending different colors in different cups, then pouring them into 1 cup without fully blending before you pour.  The result is a flowing, mottled, stone-like effect which will vary based on 5 things:

1. The difference color-to-color, high contrast will create strong patterning.
2. How you combine the colors into the pour cup, if you pour thin, thick, or varied layers.
3. If you agitate/blend the colors in the pour cup before pouring into your mold.
4. How long you wait between mixing and pouring - the longer you wait the more distinct the colors will stay.
5. The way you pour into the mold. A smooth flowing pour will look very different than a thin stream pour.

This is all demonstrated in the video below.

In this image you can see the dirty pour happening. For Rose Quartz, I was going for a softer less contrasty look, so I mixed 4 cups with different colors, but I wanted texture and separation between the colors to mimic the crystal effect, so I layered them in the pour cup without too much blending.

Try to pour level with the top of the mold.  If your casting pops out with a dip in the top, you can dome up that edge on your doming tray with either clear or colored resin, for a smooth, professional finish.

This is the complete tutorial for making a variety of Stone Effects using Brilliant Resin: