At the start of the fourth quarter, there’s one thing retailers want to know: Will Christmas sales be a lump of coal or a welcome gift? Although the fickle stock market cast a long shadow across the third quarter, retailers, manufacturers, and market watchers predict that—barring any unforeseen shocks to the economy—seasonal ­jewelry buying will give store owners a slight-to-good increase in overall sales for the holiday season and the year. As with most upbeat market forecasts, however, there’s a pinch of caution thrown in: Owing to higher material costs, sales are expected to increase while the volume of sales will most likely decrease. “With a 30 percent increase in melee prices and gold closing in at $2,000 an ounce, last Christmas’ $350 gold ring will cost $500,” says Rex Solomon, president of Houston Jewelry in Houston.“The boost in sales will bring a correspo