Here are some questions and answers you might find interesting:

(not required knowledge for students)
 

Q:  What determines the $$$ price $$$ of a ruby?

The color of a ruby is far and away the most important factor in its value.
 

Q:  But which type of red is best? and most valuable?

The rarest (and therefore the most expensive) color is a well-saturated pure spectral red--"stoplight red", free of common
secondary hues like brown, violet or purple.  Even slight deviations from this color ideal bring the price down considerably.

But color preference is subjective, so if you PREFER a pinker ruby, or one with a violet tone to it, it's just good luck that you won't have to pay as much for it!

Whatever the shade of red , it should be more "deep" than "dark".

 Color may be the most important price determinant, but clarity and cut matter, too!

As for most gems, rubies are sold by weight (carat weight), regardless of their size or measurements. Dimensions such as 5 mm X 7 mm are commonly available because they fit into pre-existing mountings

Q:  Where does this stone come from?

Certain colors may be associated with certain origins.  For example, "pinkish ruby" may come from locations like Vietnam, Madagascar and southern Tanzania.

According to the Federal Trade Commission guidelines to the jewelry industry, a jeweler who represents a gem as being
from a certain locale must be able to prove that claim with a paper trail going all the way to the mine.  This rule was implemented to prevent the common practice of using place of origin to imply value-- "Burma ruby" and Kashmir sapphire" being two designations commonly abused in this way.

While it is true that Burma is historically the source of some of the finest rubies ever produced, the fact is that the mines in Burma (like any source) produce material ranging in quality from fine gem to mineral specimen.
 

Q:  Is the color "natural", or did someone do something to it to make it look this way?

The rubies coming out of Burma today are commonly heated.  Heating can intensify the color and clarify a stone that is "silky" or cloudy looking. This treatment is permanent and non-reversable, and is one of the reasons that good, red Burmese material is more affordable than ever.