Diamonds, the other white meat.
Diamonds, they' re not just for breakfast anymore.
Got Diamonds?
Prior to the 1850's there weren't any diamonds at all. However, there were and still are locations on a table that very frequently need to be delineated, namely, that magical area we call The Kitchen. In other words, the diamonds are not just there for aiming, they originated as convenient reference points for a number of different reasons.
Early on, the only things marked on the rails were the baulk lines ( head string and foot string) and sometimes a center-line mark as well, which was typically indicated by driving a nail into the rail, no diamonds used yet.
But then the Irishman Michael Phelan came along (to the U.S.), and began studying cushion performance and rebound angles. He plotted and graphed - and tried all sorts of various cushion types and rail types and pocket types....
Long story short, he and Hugh Collender basically re-designed the entire European style table into a whole new animal, with cushions that were angle-cut at the pocket opening instead of rounded like snooker pockets. He countersunk the pocket irons into the rail itself, and then added a series of small rectangular markers into the rails, (evenly spaced all the way around the table) that indicated the usual baulk lines etc but could also be used as guides for calculating rebound angles.
The "diamond shape" was simply an aesthetic modification of the original rectangular markers and has no real purpose other than to look cool.
Figure 4 below shows the "old style" with obtuse cushions, pocket irons and marker nail above rail surface.
Figure 5 shows Phelans improvements, with the more reliable cushion style and countersunk pocket irons and rail marker