There are four amulets per character. The first one for each is already unlocked so you need to purchase the remaining three with skill points. You earn skill points by completing a set of "ravishing relics". There are 12 sets of relics (four in each set), so if you find all 48 relics during your campaign playthrough you will unlock all the skill points necessary to purchase all amulets for all four characters.
Gems are found randomly in chests. There are 9 different gems and are used to upgrade your primary weapon with different abilities.
Even if you use, destroy or sell a gem after it is acquired, it still counts as found. Chests are found in both campaign and Horde.There is a way to farm gems. In order for the gems to save, you must reach the next checkpoint. In Mission 1, 'Harbin’s Dig Site', you have an opportunity to find two gems before the first checkpoint. Start the level and run to the first chest which is down the winding pathway past the two tents. It takes about 30 seconds to get there. Shoot the pot to burn the vines to find the chest. If you don’t find a gem, you can restart the level. If you do find a gem, continue on across the bridge and enter the cave (there are enemies in here). As you emerge on the other side, pass under two arches and on the left is the second chest. Check to see if there is a gem in this chest then proceed to walk under the third arch to initiate a checkpoint then restart the level (if you so choose).
Montu's Brutal Gem: The blessings of the war god are with you. Deal increased damage to your foes!
Sekhmet's Healing Gem: Sekhmet's thirst for blood consumes your weapon, healing you with every kill!
Skull Breaker Gem: Bring the hammer down upon the skulls of your enemies. Extra damage on headshots.
Bennu's Sun Fire Gem: Set your enemies ablaze with the fury of the sun bird!
Anubis's Gem of Armourbane: The god of the dead makes your shots break through the armor of your chosen targets.
Neith's Stabilising Gem: The god of hunting lays his hands upon you and reduces your weapon's recoil.
Bast's Rebound Gem: The cat goddess makes your shots ricochet among your enemies, causing extra injuries!
Quibei's Ice Gale Gem: Freeze your enemies in their path, making them an easier target!
Quickfire Gem: The fates guide your hands. You fire and reload your weapon with greater speed.