Honestly, no. Plenty of people have managed to complete the whole game while completely ignoring the crystal system.

A character can equip 5 passive skills. Maybe 6 if you count the accessory, though accessory skills tend to be weaker than a level 3 skill. Meanwhile, if you maximize the amount of crystals per weapon, you can get 3 skills on your weapon and 3 skills on your armor, giving you 6 more skills.

A character can double the amount of passive skills they have equipped with full use of the crystal system, not to mention gaining access to some skills they normally don't have.

• Are you tired of dedicating a slot to a healer to keep your damage dealers healed every turn?
  
• Do you like seeing big numbers?
  
• Do you want to crit more?
  
• Are your characters constantly moving after the enemy?
  
• Do you want your multihitters to cripple your enemies with status effects?
  
• Do you want to solo the superboss with Sienna?

All these problems can be solved with crystals.

Besides, some of the crystals you pick up on a normal playthrough can lead to a drastic powerboost that helps you out for the rest of the game if you understand the system even if you don't want to go all out and go out of your way to farm the optimal crystals. It's not too difficult to get 2 abilities on each equipment, which will still almost double the number of abilities at your character's disposal.

• Rank: The ability of a crystal basically can range from level 1-3. For example, the ATK UP ability gives +10% at level 1, +20% at level 2, and +30% at level 3. The level 1 ability is unlocked for crystals that are Rank III and Rank IV. Level 2 is Rank V - Rank IX. Level 3 is Rank X+. Higher the Rank the better. A crystal you mine can range from Rank I - III and combining is needed to go higher. Rank I and II crystals cannot be inserted into equipment and is only useful for combining.
  
  
• Purity: Ranges from 0 - 5. The purity of a crystal is how many times a crystal can be combined with other crystals. Higher the purity the better.
  
  
• Size: Ranges from 1 - 3. An equipment has up to 4 sockets when upgraded twice. The size of a crystal determines how many sockets it uses on a weapon.

A piece of equipment actually can only have a max of 3 abilities, so putting 4 size 1 crystals will only give you the first 3 abilities. It's difficult to create a Size 1 Rank X crystals so if you manage to create any you like, I suggest you save them for the best equipment in the game since the crystals won't be the same when you remove them (more details on this later).

You can combine crystals with the same abilities. For example, you can combine an ATK UP crystal with another ATK UP crystal (with some restrictions) but you cannot combine an ATK UP crystal with a DEF UP crystal. When you perform a combination, the resulting crystal becomes artificial (has a * next to the name).

You need a Base Crystal and a Fuse Crystal for combining. There are some restrictions on the crystals you can combine.

• Base Crystal: Can be any crystal with a purity of 1 or higher (pick the crystal with the higher purity)
  
• Fuse Crystal: Can be any non-artificial crystal (think they're called natural crystals) with the same ability

Combined Result

The combined crystals will have the following stats:

• Rank: Add the ranks of the two crystals together (e.g. III + III = VI)
  
• Purity: The purity of the base crystal -1
  
• Size: Average of the two crystals' sizes rounded up (1 combined with 1 will result in size 1, 1 combined with 2 will result in size 2, 1 combined with 3 will result in size 2, etc.). This means it is impossible for a crystal to be size 1 if any size 2 or 3 crystals was ever used in the combining process.

There are a couple of key points about removing crystals from equipment:

• When a piece of equipment has more than 1 crystal inserted, all crystals are removed at once and you cannot selectively remove a single crystal
  
• The Rank of the crystal becomes III, V, or X depending on the level of the ability (Rank III for level 1, Rank V for level 2, and Rank X for level 3)
  
• The size of the crystal becomes 3
  
• The purity of the crystal becomes 0
  
• The crystal is no longer considered artificial

There are both good things and bad things about this this.

The good:

• You can freely remove and insert size 3 crystals to new equipment.
  
• You can preserve the size and rank of size 2 crystals by fusing the removed crystal into a size 1.
  
• Since the removed crystal is no longer artificial, you can use it as a Fuse Crystal again. This makes it easy to create Rank Xs without requiring high purity and high rank crystals

The bad:

• Any size 1 crystals will never be size 1 again after being removed. I highly suggest that if you have a Rank X, size 1 crystal with an ability you like, save it for the ultimate equipment.

An easy way to make Rank X crystals without requiring extremely high purity crystal and high rank crystals is to create some artificial Rank III and Rank V crystals through fusing, insert them into a piece of equipment, and then remove them from the equipment to create Rank III and Rank V crystals that can now be used as Fuse Crystals.

Usually I make a Rank III and Rank V fodder, get a crystal with purity 2 or 3, then combine those two into into it. The resulting crystal will be size 3. If I want to lower it to size 2, I add a size 1 crystal as well (if there's more purity left) and I'll have a Rank X size 2.

Example: I have the follow crystals:

• AGI UP II, Size 3, Purity 0 (Crystal 1)
  
• AGI UP I, Size 2, Purity 1 (Crystal 2)
  
• AGI UP III, Size 2, Purity 0 (Crystal 3)
  
• AGI UP II, Size 3, Purity 1 (Crystal 4)
  
• AGI UP I, Size 1, Purity 0 (Crystal 5)
  
• AGI Up I, Size 3, Purity 3 (Crystal 6)

I can turn all these into an AGI X, Size 2 by doing the following:

• Combine Crystal 2 (base) with Crystal 1 to make a AGI III, Size 3. This crystal is artificial now. Insert it into a weapon/armor and then remove it to turn this into a AGI III, Size 3 that is non artificial (Crystal 7).
  
• Combine Crystal 4 (base) with Crystal 3 to make a AGI V, Size 3. Insert and remove to create a non-artifical AGI V, Size 3 (Crystal 8)
  
• Combine Crystal 6 (base) with Cry 7 to make a AGI IV with Size 3, Purity 2.
  
• Combine the result of the above with Crystal 8 to make a AGI IX with Size 3, Purity 1.
  
• Combine the result of the above with Crystal 5 to make a AGI X with Size 2, Purity 0.

Every Crystal in the fusion process will need to be size 1. You'll need a crystal of at least purity 3 (though 4 and 5 are preferred) as the Base Crystal. I would suggest you keep an eye for crystals you're interested with a size 1 and rank II or III.

The purity of your base crystal is the number of times you can combine other crystals into it, so figure out how many fusions you need.

To get Rank X with 4 crystals, you need either:

• III + III + III + I
  
• III + III + II + II

With one of those crystals being purity 3 to act as the base for the fusions. Purity 4 and 5 crystals are more lenient. Size 1 Rank X crystals should be saved for ultimate equipment since you need 2 size 1s and either a size 1 or 2 to fit 3 abilities into a fully upgraded piece of equipment.

Each crystal node generates more crystals after 30 minutes, so you'll just have to wait 30 minutes and come back later.

I would not recommend farming crystals in the early game. As you play through the game, you can get two upgrades that make farming crystals a lot easier.

• The first upgrade lets you mine 3 crystals instead of 2 from every node.
  
• The second upgrade lets you choose whether the crystals you mine belong in the Status, Offensive, Defensive, or Utility category to narrow down the pool instead of randomly getting crystals from all 4 pools.

I highly suggest waiting until these two upgrades before going out of your way to farm crystals, but do collect any crystals if you find an active node. Might find something useful.

These are some routes that get you a decent number of crystals and requires no battles.

• Isle of Messages: Has about 10 nodes
  
• Shambala Northeast: Has about 3 nodes clumped together and is a good place to reach Isle of Messages from.
  
• Fiorwood West and Northwest: The elevated wooden bridge has about 6 nodes.
  
• Ograne Grottos: There's a clump of 4 crystals nodes in the northwest with 1 more node along the way from the quickwarp point.

Before I go into which abilities are good, do note that many abilities don't stack. For the most part, assume that abilities which don't show up on the status page do not stack. However, status crystals like ATK UP, AGI UP, etc. do stack.

Note that you cannot put two of the same abilities on the same piece of equipment even if they stack, but you can put one on weapon and one on armor.

This includes Increased Critical Damage, because critical damage seems to max at 200% and you start at 150%, so a level 3 will be enough and characters which already have it as a passive do not need it. Remember that once you remove a size 1 crystal, it will never be size 1 again so you'll need to be careful for your final setup and go for crystals that are useful for a variety of situations if you want to get 3 abilities into one equip.

For the more situational crystals, it's fine to keep a couple of size 2 and size 3s and equip and remove as needed, but I would not recommend them on a final setup with 3 abilities.

AGI UP (Status):

• Effect: Increases AGI by (15%/17%/20%)
  
• Notes: AGI is the most universally useful stat in the game and this is one of the best crystals. A character's AGI softcaps at 50 and can go higher with in-battle buffs. With enough speed + buffs, your characters can go multiple times before your enemy. One very important note is that AGI scales differently from other stat crystals and provides a whopping 15% boost at level 1 and is upgraded to 20% at level 3 while most other stats provide 10% at level 1 and 30% at level 3, so it's one of the most efficient crystals at Rank III. I try to put this on pretty much every piece of equipment unless the character can already reach 50 AGI with just 1 and their passives.

HP Drain (Utility):

• Effect: Drains HP when you deal damage.
  
• Notes: The only Utility crystal on this list and the easiest to find once you can narrow down the crystals you mine because of that. It's quite easy to create size 1 Rank X copies of this crystal. Note that this not only works for physical damage, it works for magic and summons (Cres) as well. A strong enough attacker will basically fully heal with a powerful attack if you have this. This does not stack, so make sure your character doesn't already have this as a passive.

ATK/MAG UP (Status):

• Effect: Increases ATK/MAG by (10%/20%/30%)
  
• Notes: This increases the base stats and does not include the stats gained from equipment. From my testing, it usually results in a 20% damage increase or so at level 3. Do note that at level 1, the boost is pretty meagre.

Crit Rate Up (Status):

• Effect: Increases critical rate by (5%/10%/15%)
  
• Notes: Increases critical rate. Very good for physical damage dealers and even magical damage dealers if you give them the accessory that allows magic to crit.

Crit Damage Up (Status):

• Effect: Increases critical damage by (30%/40%/50%)
  
• Notes: Increases the critical damage. Base critical damage is 150%, so with a boost, you'll be doing double damage for criticals. There's a cap at 200%, so these are pretty much not stackable.

<Weapon> Master (Status):

• Effect: Increases damage dealt by (10%/15%/20%)
  
• Notes: Increases damage dealt. This does not stack. These tend to be harder to find than the stackable stat boosting crystals.

<Armor Type> Bonus (Status):

• Effect: Decreases damage received by (10%/15%/20%)
  
• Notes: Decreases damage received. This does not stack. These tend to be harder to find than the stackable stat boosting crystals.

Call of Nature (Status):

• Effect: Raises stats at the start of battle for every free equip slot.
  
• Notes: This is a bit situation, but if your character does not really need an accessory or a class emblem, this skill will provide a nice boost to all your stats (ATK/DEF/MIND/AGI/CRIT RATE). Do note that the bonus is barely noticeable if you're still wearing 3/4 equips and is really only worth it if you only equip a weapon and an armor (or just a weapon if you have some way to avoid dying). This crystal is very rare and harder to recommend but it is an option. Personally, I would probably boost crit rate/crit damage for damage dealers instead or go for raw stats. Sienna and Mikah have this passive, so I would suggest you experiment with them before deciding to go for this or not.

Fire/Water/Wind/Earth Attack (Attack):

• Effect: Deals weakness damage and an additional (0%/5%/10%).
  
• Notes: How these abilities work is that if the enemy is weak to an element, you'll deal extra damage proportional to their negative resistance + an additional 10% (if at level 3). Now the thing about elements is that they stack, even with each other. If someone has -30% water resistance and you have 1 Level 3 Water Attack, you deal +40% damage. If you have 2 Level 3 Water Attack, you deal +80% damage.
  
  The reason I picked Fire/Water/Wind/Earth and not Light/Dark is because you can easily give an otherwise neutral target -35% weakness with Victor's Efreet's Song or A Sylph's Tale active, so if you dedicate a slot for Victor, you can make each Elemental ability a +45% damage as long as the enemy doesn't resist it. Even a +30% resistance will become a +15% damage boost. (30 - 35 = -5%, then you add the additional 10% for being a weakness). In addition to that, you can boost the damage with Oil/Dry/Wet/Heavy debuffs.
  
  Note: These only work if the skills you're using don't have an element to begin with. Elemental skills like Water Jump don't add additional elements or apply the extra damage for having Water Attack equipped against a water-weak enemy.

One thing you'll notice is that there are a lot of Status Type crystals on this list. Unfortunately, most of the other crystals are more situational, though if you have a strategy that requires one of the other crystals, feel free to play around with those as well. There are a lot of abilities in the Status Crystals pool, so it's a bit harder to get multiple crystals with the same ability. I just wanted to highlight the more safe, universally useful crystal

Another fun combo you can perform is having characters inflict Wet/Dry/OIl/Heavy (attack crystals and actives) and granting your characters the matching element with attack crystals to deal massive damage.

Crystals that have a chance to inflict stats (Attack Crystals) work best with characters that have multi-hit skills since the chance is per hit.