Affordable colored stones abound in TucsonBuyers traveling to the Tucson gem shows this month should heed a few words of advice. Price trends over the past decade suggest that larger, fine-quality unenhanced gemstones have the strongest potential to increase and retain value during a relatively short period. By their very nature, venues like Tucson offer a good chance to locate such gems at reasonable prices. This year, natural yellow sapphire and red and pink spinel are expected to be in demand. These, like many gems, are scarcer than they were just a few years ago, so watch for price differentials in similar colored gems to see if opportunities for substitution might be appropriate. For example, a 5 ct. intense yellow Burmese danburite ($250 to $300 per carat) is an attractive substitute for a similarly sized natural yellow sapphire ($1,000 to $1,200 per carat).Blue sapphire is an