Two geologists, Dr. Roeboensonsin and Dr. Thistle, were preparing a test sample for a student. The student, who was spending one college trimester doing field work, has been doing very questionable work. Both doctors wonder if he is a student or not.

The student came the next day to work through the sample. He got through every sample, finally coming to the last one. He looked down at it, and said, "These are two diamonds". Dr. Thistle and Dr. Roeboensonsin both knew that a student couldn't get that sample wrong and told him to go home, as he wasn't a student. Why was his answer wrong?