Vendor Jewels: Jewels bought Shaiya Gold from refinement vendors can be used to refine your equipment to +6, theoretically, but due to the high chance of failure and downgrade, they will typically only be used to reach +2. On average, equipment can be refined to +1 by using ~10 vendor jewels and refined to +2 by using ~30 vendor jewels.
ii. Item Shop Diamond Jewels: Jewels bought with Diamonds via the Item Shop have a higher probability of success, and less chance to downgrade upon failure. Typically, an item can be brought from +2 to +5 with ~15 jewels.
iii. Item Shop Ruby Jewels: Jewels bought with Rubies via the Item Shop grant a 100% chance of upgrade.
NOTE: Your success rate may vary; these are simply what the rates have been in my experience. It is unknown whether anything can be done to.
Success Rates and Costs - The success rates are not exact, they are really close test results. Thanks to Xelse on the more exact rates, these are the weapon success rates, and the armor rates are 5% more on each upgrade.
I don’t remember the exact costs, but the cost for weapon is much higher then the armor, I will update later with the exact costs.