Final Fantasy 15 – What Are Earth Gemstones For?

Aside from boosting your stats exponentially, there are two benefits for using the Royal Arms:

1. Deals non-specific weapon type damage, which are useful against enemies who have physical resistances. i.e. The Katana of the Warrior functions with mobility and style of a sword, but deals regular damage to enemies who have resistance or weakness to swords.

2. Enhances Warp Strike, Blitz Combos, and/or Directional Attacks to make them more powerful and effective by giving them additional abilities. iDigitalTimes Royal Arms contains a breakdown of each of the Royal Arm's hidden abilities

Sword of the Wise

1. Use as you would use a regular sword

2. Attack: 194

3. Has two-hit warp strike with delayed second hit

4. Initial hit of a Blitz combo is the strongest

5. Enables backflip as a backward directional attack

Blade of the Mystic

1. Use like a greatsword that can use combos

2. Attack: 396

3. Initial Blitz hit is slow, but speeds up with subsequent attacks

4. Warp-strike begins with three projectiles toward the enemy as Noctis stands in place, then Noctis warps forward with a physical attack

5. Pause during a Blitz combo to trigger the Tenacious self-buff that increases Noctis’ Strength by 100 for 10 seconds (does not stack but is repeatable)

Trident of the Oracle

1. Use like a polearm

2. Attack: 388

3. Creates afterimage of Noctis after each attack and performs up to three additional attacks there

4. Performs aerial diving warp-strike of up to three hits with follow-up afterimage attacks

5. Allows you to use an evasive leap and subsequent aerial dive attack as sideways or backward directional attack

Scepter of the Pious

1. Attack: 237, Magic: +150

2. Uses Magic stat in damage calculation

3. Performs two-hit aerial warp strike

4. Assumes different forms based on directional input

5. Blitz initial hit: Trident of the Oracle

6. Non-directional Blitz combo: Sword of the Wise

7. Forward directional Blitz combo: Sword of the Tall (ends in Axe of the Conqueror)

8. Sideways directional Blitz combo: Blade of the Mystic (no self-buff)

9. Backward directional Blitz combo (Trident of the Oracle)

Sword of the Tall

1. Use like a greatsword

2. Attack: 518

3. Has serrated blade that deals multiple hits per attack, with later hits dealing higher damage

4. Performs warp-strike that can deal more than 10 hits

5. Breakage multiplier of .50 is applied to all attacks but warp-strikes

Bow of the Clever

1. Uses Magic stat to calculate damage

2. Attack: 203, Magic: +80

3. Handles like a firearm with dagger-like mobility

4. Projectiles are slow and can easily miss from a distance but have a maximum range of 80 feet

5. Attacks have lower damage modifier but can pierce enemies

6. Deals bonus damage to enemies in Vulnerable condition

7. Has special warp-strike that tops in front of enemy and shoots three arrows rapidly at close range

Star of the Rogue

1. Handles like long-range pair of daggers

2. Attack: 177

3. Has multi-hit warp-strike ending with backward evasive flip

4. Allows directional aerial attacks during Blitz

5. Standard attacks pierce enemies

6. Max range of 130 feet for regular, 65 feet for warp-strikes

Mace of the Fierce

1. Use like a greatsword

2. Attack: 334

3. Has slow and linear attacks that don’t track well against fast enemies but all attacks can break

4. Performs slow to-hit warp strike

5. Great for well-timed attacks

6. All other attacks have minor damage modifier (including parries) but major breakage multipliers

Swords of the Wanderer

1. Handles like daggers when weapon is dual-wielded during Blitz combo but has weaker mid-Blitz attacks overall

2. Attack: 153

3. Has multi-hit warp strike

4. Has slower, focused hits in combined form as forward directional attack

5. Allows evasive leap with subsequent aerial dive attack as sideways or backward directional attack

Shield of the Just

1. Handles like regular shield, including warp-strike

2. Attack: 251, Strength: -100

3. Hold square or x to enter defensive stance similar to taking cover. Generates same HP but drains MP

Sword of the Father

1. Handles like Sword of the Wise but has unique attack animations such as a three-hit warp-strike with follow-up afterimage attacks

2. Attack: 141, Strength: +100

3. Enables unique Tenacious self-buff via a Blitz finale, increasing Noctis’ Strength by 200 for 5 seconds

Axe of the Conqueror

1. Use like a greatsword

2. Attack: 483, Strength: +60

3. Performs aerial dive warp-strike

4. Has slow, powerful, linear attacks

5. Enables aerial diving attack as forward directional attack

Katana of the Warrior

1. Handles like sword but has extended range and faster Blitz combos after initial hit

2. Attack: 361

3. Has delayed single-hit warp-strike and lets you chain successive warp-strikes quickly

There are, in fact, certain treasures that are used in quests or to upgrade weapons (also considered quests).

I found a list of all of these "Don't sell" treasures in the game here:

• Barbed Scythe
• Cactuar Needle
• Coeurl Whiskers
• Cylinder
• Dynamo
• Earth Gemstone
• Glass Gemstone
• Magitek Core
• Magnetron
• Metal Scrap
• Monster Claw
• Repair Kit
• Rusted Bits
• Sky Gemstone
• Spiked Armor
• Sturdy Helixhorn

Thus, it looks like all other treasures are able to be sold without worry.