Bonmoyer, I'm so sorry you had that experience with DI. I would be sick too!
I was fortunate to have had a good experience with Tanzanite International (same company as DI) two years ago. With great trepidation, but a lot of excitement, I bought an emerald and diamond ring in Cozumel. When I returned, I immediately had it appraised at a reputable appraisal service. The appraisal confirmed the quality of the gems and the appraisal price was just under four times what I had paid.
I guess, like anything else, some experiences are good and some are not.