A traditional name for a very small size of metal type, standardizing to roughly 4.5 points in the American point system. It sits between brilliant (4-point) and pearl (5-point) in the old naming convention. Because of its extreme smallness, it was rarely used for continuous reading text, being reserved for pocket bibles, footnotes, and compact reference works.
Heard on the job
You'll go blind trying to set a whole column in diamond, it's barely legible even with a magnifying glass.