The Flawless Skull is a powerful gem in Diablo 4 that can be crafted and applied to socketed equipment. The Flawless Skull grants different bonuses depending on the type of equipment used, thereby enhancing your character's abilities. This is a complete guide for Flawless skull gem which includes its effects, upgrading process and usage:

Effects of Flawless Skull:

Flawless Skull gem provides unique benefits depending on the kind of equipment it is put into:

Weapons: When you attach it to a weapon, the Flawless Skull allows you to get life upon kill bonus. The actual value of this bonus varies between [0.036-~], giving you extra life regeneration per slain enemy.

Armor: Survival ability of your character will be improved if you put these skulls in your armor. It gives up to [7-1]% Healing Received bonus. You are going to recover from any healing effects much better while fighting.

Jewelry: If it's a ring or an amulet, the Flawless Skull will give +[295] Armor as an additional characteristic. Your defense against incoming blows will increase by +[295] Armor points.

Upgrading Flawless Skull:

You may enhance the effectiveness of Flawless Skull gem through upgrading it into higher tiers. But it has one requirement – investing 3 Gems and Gold each time; however, its strength increases significantly when used with other gear slots. There are no known specifics about development after Royal skull level.

Crafting Flawless Skull:

To craft a flawless skull gem, gather the following resources:

3 SKULLS: These materials can be obtained by killing monsters, looting chests or other means available within the game world.

18 000 GOLD: You can loot treasure, do quests or sell items in order to obtain necessary quantity of gold.

Once you have obtained all the necessary materials, you can create flawless skull gem by using the services of the Jeweler NPC.

Socketing Flawless Skull:

Flawless Skull can be inserted into any socketed equipment among weapons, armor and jewelry. This allows you to change your gear and customize your character's abilities according to your playstyle and preferences.

If you want to switch your gear combinations or try out different strategies, the Jeweler can remove gems from your gear without destroying them. Therefore, you can reuse Flawless Skulls on multiple items of equipment that you possess and thus optimize your character's build for various cases.

Note that this information about Diablo 4 and Flawless Skull gem is contained in these texts based on what you have provided me with and does not involve any changes or improvements that could have taken place in the game since my knowledge cut-off date in September 2021. For accurate and latest information please refer to official Diablo 4 sources, as well as in-game documentations.