How to reproduce faux tortoise shell material
I've recently viewed a YouTube'r creating pen blanks with two different colored resins and pine cones. She placed a timer in the camera view and after mixing the two parts of the respective resins waited for a specific time before pouring the resins into the mold.

I did not pay sufficient attention to the elapsed time and it would depend on the resin being used. I've seen resins that gel up in three or four minutes and another product that has forty-five minute working time.

You may be able to use your existing resins if you are able to determine the edge of the gel period.