Help to ID blockage material

Background: We live in a rural area with a septic system. The standing rule is NOTHING solid goes down the drain, period.
We have an island sink in addition to the kitchen sink. About two or three years into our new home water backed up and spilled out of the island sink vent while the kitchen sink was being drained, so I knew the source of blockage was downstream of the "Y" where both come together. I got a manual cable snake and out came a white,viscous, granular grease like material---almost like shortening, but with no odor. Very strange, I thought, but probably easy to manage. So I initiated a PM program at the end of every month when we crank up the hot water heater and run a couple of sinks of hot water down the pipes to keep things moving. That worked for probably 6-7 years, until now.
During last weekend's maintenance the island sink vent overflowed which surprised me. I ran the clean out cable down and hit something just downstream of the "Y" for both sinks. Once I thought I had dislodged the obstruction I ran hot water and monitored the outside clean out and caught these pieces of solid material on their way out. My wife's first reaction was that is was dishwasher soap powder, but I find it difficult to believe that in the wet confines of a drainage line powder would dehydrate back into a solid. The material is homogenous, if you break it open it is a solid, white, chalk like material with no organics or others embedded in it. If you assemble these pieces they form a perfect 2" cylinder. The ID of the blockage material is pockmarked with numerous small craters which is indicative of erosion, not deposition. Further the ID of the build up is coated with black slime, which I think would not be conducive to additional deposition. This would almost lead me to believe that this was possibly there when the home was built, but I don't know how I could have missed it the first time. If it was subsequent to our move in I want to identify the root cause to prevent it in the future. Any comments or ideas, anyone? Thanks!
Michael Arvanetakis
Cypress, TX

Kitchen sink... its grease years of it and everything else that went down that drain

Thanks, but we do not run any grease or oil down the sink---standing house rule! And it is a brittle, solid, chalk like substance---it is not a semi-solid. It crumbles when crushed. Can't find any fibrous material in it, so it isn't paper or cloth lint. Also: All the pieces retrieved were about the same length, indicating the original obstruction was apparently clearly defined in a limited space.

Do you use your kitchen sink or dishwasher to wash dishes? Then yes, you put grease down the drain. Most people know better these days than to just dump your food and grease down the drain, but there's still residue on your dishes, it adds up over time, it's there.

From what you've described, it fits the bill for kitchen sink grease perfectly. The powdered dishwasher detergent could also be a factor, since once that gets a chance to sit it wants to go back to a solid state. The liquid packs do not do this. This is why I've eliminated bar soaps and powdered detergents from our household.

I think Matt is pretty accurate in his explanation.

You would not believe how many times I have heard " We don't put any solids or grease down the drain. But yet I'm there snaking it out.
being on a septic is probably wise avoiding solids, but I tell my customers not on septic to use the disposal with moderation. Like when the doctor tells you to eat more fiber. The solids help wipe of the oils and sticky residue.
Ever try to take a shower with out rubbing yourself. Standing in a shower with just the water running over you doesn't work very well. or just washing your hands. you have to rub them to get them clean.

Voletl, Matt and David thanks for your comments and points well made. Matt, you have made some good points and we are going to liquid detergent for the dishwasher. Thanks all!