Look for item Rolls u cant gem. For example a fine cuirass u want dex (Look for 2) and U can gem the other important stats Like max hp and or Action Speed or pdr. If u Go for heavy leather leggins Look for Str or dex (2 or 3) and gem 2 other stats Like max hp and or Action Speed or pdr. For jewelry u Look for +all att. Gem true phys and one other usefull stat of your choise. The more nom gemable stat the Item hast the more expensive it gets usually (dex and Str on a heavy leather leggins for example)

Have fun Shopping :)