Arms Augment Gems

An arms augment is a small gem, crystal, or similar object that provides a magical effect when affixed to a weapon. Each item can hold a single augment crystal, but an attached crystal can be swapped for another one at any time. Attaching an augment crystal to (or removing it from) an item requires a move action that doesn’t provoke attacks of opportunity. Effectively, each eligible item has a single “slot” that can be filled by any appropriate augment crystal.

Crystals can come in 3 forms, Lesser, Moderate and Greater. You must meet the level of a crystal to be able to use it. You get the effect listed for your rarity of gem plus the effects for the lower rarity gems. (ie. a greater gem gets the moderate and lesser effect)
Lesser Gems – 3rd Level
Moderate Gems – 9th Level
Greater Gems – 15th Level

NOTE – 2 indentical gems can be fused to create a gem of the rarity up. (ie two lessers make a moderate.)

Gem of Arcane Steel

A crystal of arcane steel is designed for those who can blend magical and martial arts into a single strike. This crystal grants:
Lesser – You get a +1 enhancement bonus on your weapon attack and damage roll when delivering a spell or spell-like ability through an attack with the weapon.
Moderate – You also increase the save DC of the spell or spell-like ability by 1, when using this weapon.
Greater – You gain an additional +1 enhancement bonus to spell attack, damage and save dc when using the weapon.

Gem of Energy Assault

A gem of energy assault radiates with the power of the elemental planes. They grant benefits depending on the colour of the gem (red-fire, blue-cold, green-acid, yellow-lightning, black-necrotic, white-radiant).
Lesser – You add 1d4 point of the associated energy damage to the weapon’s damage.
Moderate – Instead add 1d6 of the damage type to your attack, and it now stacks with other sources of the same elemental.
Greater – You also gain a secondary effect depending on the type of crystal.

Secondary Effects:
Acid Assault
Target takes –1 penalty to AC for 1 round.
Cold Assault
Target’s speed is reduced by 10 feet for 1 round, to a minimum speed of 5 feet.
Fire Assault
Target takes an additional 1d6 points of fire damage 1 round later.
Lightning Assault
Target takes a –1 penalty on attack rolls and perception checks, for 1 round.
Necrotic Assault
Attacks the target makes have disadvantage until the end of its turn.
Radiant Assault
Attacks against the target have advantage until its turn.

Gem of Enhancement

A gem of enhancement is a simple gem, it is empowered with the same magic given to magic weapons. However this enhancement stacks with others of its kind.
Lesser – Your weapon is now considered magic, it gains a +1 to attack and damage.
Moderate – Your weapon gains an additional +1 to attack and damage.
Greater – Your weapon gains an additional +1 to attack and damage.

Gem of Life Drinking

This gem hungers for blood and rewards you for feeding it. each time you deal damage to a living creature with the attached weapon, the gem heals you. Dealing nonlethal damage with the weapon doesn’t activate the crystal’s effect
Lesser – You heal 1 hit point per hit.
Moderate – You instead heal 3 hit points per hit, however you can no longer have temporary hit points.
Greater – You instead heal 5 hit points per hit, and still cannot have temporary hit points.

Gem Of Return

A gem of return allows a weapon to leap into its owner’s hand when called.
Lesser – Drawing this weapon is a free action, addition you can call the weapon (if unattended) to your hand from up to 30 feet away as a move action.
Moderate – You may ignore the reload property, or the weapon now returns to your hand automatically at the end of the round.
Greater – Your ranged ammo now returns to you if undamaged, or the weapon returns to your hand as a free action.

Gem of Slaying

Gems of slaying were designed for those who fight the same type of creature on a regular basis, most often they are used against constructs or undead. Each gem is attuned to a different creature type (aberration, construct, dragon, elemental, fiend, or undead.)
Lesser – The weapon deals an extra 1d6 points of damage to the chosen prey.
Moderate – This weapon can also deliver sneak attacks and critical hits against those normally immune, OR it is able to overcome a single immunity of the chosen prey.
Greater – The weapon now auto crits against the chosen prey.

Gem of Weightlessness

This gem makes your weapon lighter and easier to wield.
Lesser – Removes the heavy property from your weapon.
Moderate – Removes the two handed property from your weapon, and removes the reload property from ranged weapons.
Greater – Adds the light property, and you can make a single attack as a free action each round if dual wielding (with each equipped weapon that has this gem in it).

Image – https://lootthebody.org/2018/03/08/return-of-the-weapon-gem-5th-edition/