Hi, Welcome to health care magic, Eating spinach might cause slightly geenish stools but as said you had also black coloured stool and olive sized balls but it requires further evaluation by some blood and stool tests to detect the cause. It can also occur in various conditions like excess unprocessed bile (which is to be digested by certain enzymes)or iron supplements,or iron riched diet,celiac disease or Protozoa infestation etc. So,you should go for complete hemogram,urine and stool examination with liver functions tests with ultrasound abdomen to evaluate it further. You need to see physician for complete medical history and thoroughly clinical examination and relevant tests reports. Regards,