The book Star Wars Lightsabers: A guide to weapons of the force indicates that Luke's green lightsaber uses a "synthetic" green crystal as its focusing crystal while all other Jedi in the book use some form of Adegan crystal.
Furthermore, the text claims that generally only the Sith use synthetic crystals and they form these by focusing their anger and hate during the crystal forming process.
Given this information, how and why did Luke manufacture a synthetic crystal, and does this affect his use of the light-side force?
EDIT: Thank you to those who have noted that information in this book has now been excluded from official cannon, however for this question I do not require that the answer be restricted to official canon sources. Any information from the Star Wars Universe is relevant (excluding the fan-fic you wrote in middle school of course, no matter how awesome your cousin thinks it is!).