Marble effect pigment for injection molding
The marbling effect or tortoise shell effect developed in the last century was typically achieved by adding 2 or more different colors of the same basic resin and processing through a plunger style machine versus a reciprocating screw machine as is commonly found today. The reason it worked was because that type of injection molding machine did not mix the materials very well and you would have streaks of the dissimilar colors showing on the surface of the product. A similar effect can be achieved by removing a section of flight from the mixing section of a general purpose screw for a reciprocating screw injection machine. To my knowledge, there has never been a true "Marble Effect Pigment" that gives the effect every time with any type of equipment.