Jewel rates:
Jewel of Bless: 100%
Jewel of Soul: 70%
Jewel of Life: 80%
Combination rates:
Item level +10: 75%
Item level +11: 70%
Item level +12: 65%
Item level +13: 60%
Item level +14: 55%
Item level +15: 50%
Failed combination = Items burned
Item +luck option = increases +10%
You can use the Talisman of Chaos Assembly to prevent your items from disappearing in case of failure. To increase the success rate, you can also use the Talisman of Luck. Both of these items are available in the XSHOP.