Rubies are red with a medium to dark tone. Pink is a lighter saturation of red, hence the limitation of red as a medium to dark color. A light-toned ruby is by definition a pink sapphire.
Beyond this definition, rubies and pink sapphires are the same thing: corundum (Al2O3) colored by chromium, with corundum being the mineral species of rubies and sapphires. The only difference is the levels of chromium present coloring the corundum. GIA grades the tone on a scale from 2-8.
Saturation, how brown or red the color is, is also a requirement to be a ruby. However, rubies that have any brownish coloration in them are exceedingly difficult to sell and most gem sellers will not bother with these types of rubies.
It is easy to determine corundum as pink or red if the material is very light, or very dark and saturated. However, there are plenty of stones that are borderline cases between “ruby” and “pink sapphire”.
P3367 | “P3367”P4292| “P4292”P4357 | “P4357”
P4353 | “P4353”P4310 | “P4310”P4442 | “P4442”
P4028 | “P4028”P4413 | “P4413”
There is another layer in the debate: Faint blue traces that make these pink sapphires purplish also help make their tone darker to possibly qualify as a ruby. Many times when the purplish traces are removed, their tone is lightened and the gems cannot qualify as rubies.
Due to this indeterminate border, some ruby vendors will buy pink sapphires and sell those same stones as rubies. The price difference in pink sapphires and rubies is a stark comparison too, regardless of the quality of the pink sapphires. Very often the most vivid and saturated pink sapphires are borderline rubies, and can qualify as rubies in places like Sri Lanka and Myanmar (formerly Burma). It is difficult enough to get a consistent standard of what qualifies a ruby in place, but having it internationally agreed upon is another level of difficulty altogether.
There are other similar issues in the industry over other colors like padparadscha sapphire, or the border between emeralds, aquamarines, and green beryl since they are all varieties of beryl with specific coloring.