Ideally, to be competent in gemstone grading, you'll need to have several years of market experience in buying/selling gemstones, as well as a good formal education in grading/appraising gemstones. Since you will eventually learn from handling stones, I'll talk about getting a formal education.
Many people choose to go to GIA to get an education in gemstones. That's good because GIA is reputable and will help give your clients confidence in your knowledge. However, some of the best places to get a gem education are the least well-known.
I used to work for Asia's oldest gemological institute: The Asian Institute of Gemological Sciences. It is headquartered in Bangkok, the gem capital of the world. It has an excellent diploma prgram that includes gem identification, synthetic and treated stones and colored stone & diamond grading and appraisal. The true value-add in going to AIGS is not just the established program it offers, but the huge collection of gemstones available to you for practise. Not even GIA offers as many stones or hours in the lab for practise. The added plus point of going to Thailand, is the unique opportunity to network and make contact with the many gem dealers here in Thailand, as well as others who use Bangkok as a base - eg. dealers from India, China, Burma, Sri Lanka, Africa etc.
As to reputable places to buy stones, well - I'm not sure where you are and can't really answer that. I can say this however, Bangkok has a plethora of gem shops and dealers who can supply you with any gem you need and at any price category. Chances are that the person you buy your stones from coms to this part of the world to source for his.
There is no one system in use with regards to the grading of gemstones. (This leaves things open to general market ideals and to one's personal taste which isn't a disadvantage in my opinion.) In general, the purer and more saturated the color, given that tone is balanced and cut is desirable, the higher quality the stone is deemed to be. Premiums are given for natural material, although this also largely depends on the type of stone and its variety. Treatments of any kind should be disclosed. Again, whether it lowers the value depends on the type of stone.
For a rough guage on prices b/o quality and also basic information on treatments, try "The Guide". It publishes wholesale gem prices in the US. Check www.gemguide.com for subscripions and more information.
I'm sorry this was "brief". It really is hard to answer this in detail without filling up the whole page! *lol* In any case, I hope that helped!
I LOVE gemstones and am a gemologist.