Rocks in Washing Machine?

We recently purchased a home that had the washer and dryer left behind. The washer is a GE and in good condition. We were told by the previous owners that the washer worked fine, but we tested it with a couple loads of rags to be sure.

It seemed alright so we did two loads of our laundry and on the second load it wouldn’t drain. We kept getting a warning buzz and blinking lights. Code read clogged drainage hose so we took it apart and sure enough it was all jammed up….with ROCKS.

They looked almost like small pieces of concrete that had been soaked for a while. Many of them looked like they had originally been formed into a square grid. There were about 30 pieces total.

I cleaned the lines and put everything back together. So far the test loads have been running fine.

My questions are…

1. Are there any parts in a washing machine that would splinter into rocks/stones when breaking but still allow the washer to work?
2. Is there something else I should be checking or am I correct in assuming the previous owners children may have had rocks in their packets when their cloths were washed?