"Crystal Beast", known as "Gem Beast" (宝玉獣Hōgyokujū) in Japan, is an archetype of Monster Cards that all share the effect of becoming Continuous Spell Cards upon their destruction. Although weak in terms of actual strength, their aforementioned effects allow them to have great staying power. In addition, they are supplemented by many support cards which utilizes them in their Continuous Spell Card form. When all seven Crystals are on the field or in the Graveyard, their boss monster, Rainbow Dragon, can be Summoned. This archetype also contains one of the most powerful OTK setups with "Crystal Abundance", and even sturdy control with the five effects of their field spell.
The "Crystal Beasts", whose original name is "Gem Beasts", as their name says, are all animals which have each a different kind of gem in their bodies. Their different gems' colors form the seven colors of the rainbow, reflecting the name and design of their ace, "Rainbow Dragon". Their nomenclature is "Crystal Beast (gem) (animal)", and their attack names are "(gem) (part of their body/attack)".
"Crystal Beast" Decks tend to have a wide variety of searching options for putting "Crystal Beast" cards on the field, either Summoned as monsters or placed in the Spell & Trap Card Zone as an effectless Continuous Spell Card, and they have a couple of extra drawing options to speed up the use of the Deck. Because of these effects working together, "Crystal Beast" users can wind up many cards ahead of their opponent, and the various searching options facilitate the Summoning of "Rainbow Dragon" far quicker than would otherwise be possible. The archetype also supports come-from-behind wins quite easily due to the double whammy of "Crystal Abundance", which wipes out everything on the field and then swarms the field with monsters from the Graveyard, and the Field Spell Card of the set, "Ancient City - Rainbow Ruins", gains a new effect for every Continuous Spell Card "Crystal Beasts" you control. The archetype is also somewhat resistant to opponents who use Spell and Trap destruction effects, as a couple of cards can quickly put the "Crystal Beasts" in Spell form back onto the back row from the Graveyard, and the Field Spell Card resists being destroyed itself.
It should be noted that, although "Rainbow Dragon" is the boss monster of this archetype, its inclusion is more likely to be a great monster, you need a good strategy to get Crystal Beasts out in one turn; maybe two turns if your hand is not good. Because of this, "Hamon, Lord of Striking Thunder" is often used in its place, but "Malefic Cyber End Dragon" avoids both the lengthy set-up and the hefty costs of the previous bosses, and boasting the same 4000 ATK but it can be removed easily from the field due to the fact that it gets destroyed once your Field Spell card is removed from the field. Malefic Cyber End Dragon requires a Field Spell to be Summoned and is the only monster to attack once it is on the field, so you may not want to put this card in your Deck.
With the introduction of Xyz Monsters, "Crystal Beasts" have become a more difficult Deck to use. With impressive swarming abilities, a "Crystal Beast" player can easily Xyz Summon two or three times in one turn. However, when these Xyz Monsters are destroyed, all of those "Crystal Beasts" will be trapped in the Graveyard, requiring slow pick-up from "Sapphire" Summons, or risking "Crystal Blessing", which will not always have targets. The key is to Xyz Summon only when absolutely necessary, as excessive overlaying will exhaust the Deck's longevity in no time.
If a card such as "Macro Cosmos" is preventing cards from reaching the Graveyard, this will NOT prevent "Crystal Beasts" from using their effects (as they would never hit the Graveyard in the first place).
The OTK is more competitive friendly. It utilizes the effect of "Crystal Abundance" to work, so a few copies of the card is essential to have in the Deck, as well as a way to search it out ("Different Dimension Capsule" or "Gold Sarcophagus" work nicely). There must be four Crystal Beasts in the Spell & Trap Zone, plus an open slot to activate "Crystal Abundance". Wait until you have five Crystal Beasts on the field or in your Graveyard with ATK of 1600 or more to activate the card. Upon activating "Crystal Abundance", if your opponent had at least five cards on his/her field, all five of your monsters can be Special Summoned. "Crystal Abundance" leaves your opponent open to a direct attack. Another way to achieve this outcome is by the use of the Destiny Heroes. With the combination of the various normal spells and "Destiny Hero - Diamond Dude", it can make a speedy Deck. A great way to speed up the process for "Crystal Abundance" is to play "Crystal Tree" because you just need to play it and ensure that two "Crystal Beast" are destroyed and become "Crystals" and then use "Crystal Tree" to finish the other required half for "Crystal Abundance. Just be careful with Tree as it has an annoying side effect: if it has more counters than you have spaces, you can't send it to the graveyard. As soon as it gets to 5 counters, you can't get rid of it. Keep "Mystical Space Typhoon" handy just in case.
The management of maintaining vacant Zones in the Spell/Trap Card Zones can be irksome sometimes for these kind of Decks due to the balance between "Crystal Beast"s in said Zone for certain support cards as well as regular Spell/Traps to help this Deck further, a possible annoyance that can be hustled via cards like "Xing Zhen Hu", "Xing Zhen Hu Replica" and "Seismic Shockwave".