Ratnapura is the center of Sri Lanka's gem mining industry. The mines are worked in the dry season and the gems found are the ruby and sapphire (plain and star varieties), cat's eyes, alexandrite, topaz, amethyst, aquamarine, tourmaline, garnet, zircon and a host of others.
Ratnapura is not a particularly distinguished town in anything but it's gem trade. It is here that people gather from all the gemfields across Sri Lanka to trade the yields of their hard labour. Wander through the main market area and mention the word sapphire and the dealers will emerge as if from no-where, tiny paper envelopes full of stones miraculously appearing from the pockets of stunningly bleached business shirts and thrust in front of you for inspection.
You could fend them off with a big stick, or enjoy the tumult of it all, suggesting a price here and there, haggling for 1/5 of what you expect they'll want for a particular parcel of stones. All good fun, but learn a bit about gemstones before you head off.