@Glasshouse- The bureau of labor and statistics groups gemologists in the same employment category as jewelers and stone cutters, so there I could not find much reliable data on only gemologists. According to the bureau of labor and statistics, the job opportunities for these professions is growing slower than normal, somewhere around five percent per year. The more training one has, the less competitive the job market.
Your assumption that the industry sheds jobs during an economic downturn is correct. The jewelry industry does not sell necessities so the industry can be cyclical. The ups and downs in the cycle can also be hard to predict and can last a while.
If you were planning on pursuing a degree in gemology, it would be wise to major or minor in a related field. The wages for jewelers and gemologists can be lower than most other physical scientist jobs, so you may need to broaden your education to ensure that your degree is a solid investment.