The idea of birthstones seems to go pretty far back but I’ve yet to find much trustworthy information on the whys and wherefores. In 1870 Tiffany & Co. published a poem with the traditional birthstones of English speaking countries, which sort of codified them. As new gemstones are discovered the calendar is sometimes modified to include them. (For instance adding pink tourmaline to October-why couldn’t they have left well enough alone? And tanzanite to December-now that’s an upgrade!)
April has remained steadfastly diamond.
“She who from April dates her years,
Diamonds shall wear, lest bitter tears
For vain repentance flow; this stone,
Emblem of innocence, is known.”