Diamonds can be set in any metal, not just 18 karat.
Most typical are diamonds set in 14 karat gold actually. Due to market demands and the rising cost of gold, there are a lot of new jewelry designs that have diamonds set in silver.
I'd assume that if the diamonds are set in 22 karat gold, they are most likely flush set. The first image on the following link is exactly that: http://kenthibado.com/gallery.html
22 karat gold is very soft, and wears away faster than golds of lesser karat. If a client of mine asked for a 22 KT setting, I'd insist that they use a bezel, but I wouldn't talk them out of the choice of metal.
Hope that helps.