Hi Willy, "Gemstone" is a much stronger call-sign than "Emerald":
Two syllables instead of three, so quicker and easier to say in busy airspace.
The "J" sound of 'gem' and "St" sound of 'stone' will be much clearer and will stand out much more over a noisy RT than the much softer sounding 'emerald'.
Having been met with blank looks and incomprehension when I tried to persuade a previous airline to use better call-signs and with fewer syllables; I am sure that none of the above were management considerations though !