Giant Topaz was added in last update. You can loot it from few bosses from the Feaster of Souls Quest.:
- Brain Head
- The Unwelcome
- the Pale Worm
Those bosses are confirmed on TibiaFandom and tibiaBr, but I think we can also loot it from those bosses:
- The Dread Maiden
- The Fear Feaster
TibiaFandom - link
tibiaBr - link
• Goshnar's Greed
• Goshnar's Malice
• Goshnar's Megalomania
• Goshnar's Spite
• Tentugly's Head
Source : Tibiopedia.pl