Tablet

Tablets are stone slabs with inscriptions etched into their faces, hidden throughout Weyard in Golden Sun: The Lost Age.

These mythical stones have the power to teach their finder either dual elemental summons like Megaera or Moloch, or an elemental Utility Psynergy; the former type is far more numerous than the latter. Most Summon Tablets can be found either in short caves or at the end of small side-dungeons. Psynergy Tablets are found within each of the four Elemental Rocks scattered across the world. Often specific Psynergies are needed in order to reach the tablets.

Summon Tablets
In typical order of acquisition:

Zagan: Zagan's Summon Tablet is found in Indra Cavern, a cave found north of Madra. The Move and Lash Psynergies are needed to obtain it. This is the first Summon Tablet available, and so will probably be the first multi-elemental summon a player obtains. Megaera: Megaera's Summon Tablet is found in Osenia Cavern, a cave found northeast of Mikasalla. The Scoop Psynergy is needed to obtain it. Flora: Flora's Summon Tablet is found in the third room from the top of Air's Rock and cannot be obtained until after the guardian statues are activated. In order to reach it, a stone pillar must be pushed so that it is carried away when the player casts Whirlwind on a stone on the other side of he room. Once moved, the pillar can be pushed into such a position that it serves as a stepping stone used to reach the Summon Tablet. Moloch: Moloch's Summon Tablet is found in the Madra Catacombs. Although the catacombs can be visited when first arriving in Madra, it cannot be fully explored until Piers joins the party and Sheba learns Reveal. The Summon Tablet is locked behind a door that can only be opened with the Ruin Key. The Tremor Bit can also be found in the catacombs, which is convenient as Tremor is needed to reach the chest containing the Ruin Key. Ulysses: Ulysses' Summon Tablet is found in the Izumo Ruins. Although the catacombs can be visited when first arriving in Izumo, it cannot be fully explored without the Sand Psynergy found in nearby Gaia Rock. Parch is also needed to reach the Summon Tablet, meaning players who have not already been to Aqua Rock will have to backtrack to obtain this Summon sequence. Eclipse: Eclipse's Summon Tablet is a prize at the Lucky Medal Fountain in Lemuria. It is won the first time a Lucky Medal in the center of the spring. This is arguably the easiest Summon Tablet to obtain. Haures: Haures' Summon Tablet is found in Angara Cavern, a cave found to the north of Loho. The Carry Psynergy is needed in order to obtain this tablet, meaning it cannot be reached until after Isaac and Felix's parties team up. Coatlicue: Coatlicue's Summon Tablet is found in Atteka Cavern, a cave in the southernmost part of Atteka. The Parch Psynergy is all that is needed to obtain this tablet. However, the cave itself cannot be reached until after the Wings of Anemos are attached to your ship. Azul: Azul's Summon Tablet is found in the back of Treasure Isle. Although the island can be entered as soon as the party first sets sail, the Grind and Lift Psynergies are needed to fully explore the island. This is the first of the "ultimate" summons that can be obtained, since the Teleport Psynergy is not a requirement. Upon reaching the tablet, the Star Magician appears and must be defeated before Azul can be acquired. Daedalus: Daedalus' Summon Tablet is found in the back of the Yampi Desert Cave. The Sand Psynergy is all that is required to reach the cave, although it cannot be truly explored without the Teleport Psynergy. Upon reaching the tablet, Valukar appears and must be defeated before Daedalus can be acquired. Catastrophe: Catastrophe's Summon Tablet is found in the back of the Islet Cave found in the Sea of Time Islet. In order to enter the cavern the player must complete the game's trading sequence. However, it cannot be fully explored without the Teleport Psynergy. Upon reaching the tablet, the Sentinel appears and must be defeated before Catastrophe can be acquired. Charon: Charon's Summon Tablet is found near the entrance to Anemos Sanctum, which cannot be entered without all 72 Djinn. This requires players to obtain all 28 Djinn from Golden Sun and then transfer their data to The Lost Age. As a result, not all players will be able to acquire this Summon sequence. If one does succeed in entering the dungeon, the summon is easily obtained. Iris: Iris' Summon Tablet is found in the back of Anemos Sanctum, which cannot be entered without all 72 Djinn. This requires players to obtain all 28 Djinn from Golden Sun and then transfer their data to The Lost Age. As a result, not all players will be able to acquire this Summon sequence. Iris is guarded by Dullahan the strongest enemy in the entire game, who must be defeated in order to acquire the Summon Tablet. Since there are likely no bosses remaining after Dullahan's defeat, Iris is meant more as a trophy or a "bragging rights" summon.

Psynergy Tablets
There are four tablets that bestow a Utility Psynergy to Felix and each of his party members:

Parch Tablet: Found within Aqua Rock. Allows Piers to dry bodies of standing water within towns and dungeons. Reveal Tablet: Found with Air's Rock. Grants Sheba the ability to perceive hidden truths within towns and dungeons. Sand Tablet: Found after defeating the Serpent within Gaia Rock. Allows Felix to turn into sand and traverse conduits of it within dungeons. Blaze Tablet: Acquired within Magma Rock. Allows Jenna to amplify small flames.

In Golden Sun: Dark Dawn
Zagan: Zagan's Summon Tablet is found in Carver's Lumberyard, an area found south of Carver's Camp. The Whirlwind Psynergy is needed to obtain it. This is the first Summon Tablet available, and so will probably be the first multi-elemental summon a player obtains. Zagan Cannot be obtained after Blados seals the exit of the Konpa Ruins! Megaera: Megaera's Summon Tablet is found in the Konpa Shrine, a small cave found west of Harapa. The Douse Psynergy is needed to obtain it. Flora: Flora's Summon Tablet is found inside a hidden room inside the Passaj Mountain Climb and cannot be obtained until after the Ice Queen is defeated. In order to reach it, an ice pillar must be formed with Cold Snap so that the second piece of Zol is carried into the hole when the player pushes it. Once both pieces of Zol are in place, the door to the Summon Tablet is opened. Moloch: Moloch's Summon Tablet is found in the North Wall Shrine, Northwest of Craggy Peak. After beating the Craggy Peak Ruins visited when leaving Passaj. Moloch Cannot be obtained after clearing Luna Tower. Ulysses: Ulysses' Summon Tablet is found in Border Town Mine. The player must change the tracks to reach a bomb lodged in a wall. Use Fireball Psynergy to ignite the bomb, causing it to explode, revealing the Summon Tablet. Ulysses Cannot be obtained after clearing Luna Tower. Eclipse: Eclipse's Summon Tablet is a at the top of Luna Tower in Belinsk. It is at the top of the staircases held inside of a orb shaped like the moon. Eclipse Cannot be obtained after leaving Luna Tower. Haures: Haures' Summon Tablet is found in Border Town, but a Dream Leaf from Kolima Village is needed to obtain it. The Growth Psynergy is needed in order to obtain this tablet. Haures Cannot be obtained after clearing Luna Tower. Crystallux: Crystallux is obtained after leaving Luna Tower, when Volechek and his men go into the Belinsk Opera House, where they have imprisoned a Dark Devourer and two Curse Scuttlers. After defeating them, Crystallux turns himself into a Summon Tablet. Crystallux Cannot be Obtained after boarding Briggs's ship. Coatlicue: Coatlicue's Summon Tablet is found in Harun Channel, a cave southwest of Warrior's Hill, which must be completed first. The Move and Crush Psynergies are all that is needed to obtain this tablet. Azul: Azul's Summon Tablet is found onboard the Lost Ship, located in a shroud of Purple Fog on the world map. This is the first of the "ultimate" summons that can be obtained, since the Sol Blade is not a requirement. Upon reaching the tablet, the Star Magician appears and must be defeated before Azul can be acquired. Daedalus: Daedalus' Summon Tablet is found in the back of the Burning Island Cave. The Search Psynergy is all that is required to reach the cave, although it cannot be truly explored without the Sol Blade. Upon reaching the tablet, Ogre Titans appear and must be defeated before Daedalus can be acquired. Catastrophe: Catastrophe's Summon Tablet is found in the back of Otka Island found west of Yamata City. In order to enter the cavern the player must have Himi in the party. However, it cannot be fully explored without the Sol Blade and knowing the solution to the maze. Upon reaching the tablet, the Ancient Devil appears and must be defeated before Catastrophe can be acquired.

Charon: Charon's Summon Tablet is found on the last floor of Crossbone Isle, which cannot be entered without beating the game. If one does succeed in entering the dungeon, the summon is on the left side. Iris: Iris' Summon Tablet is found on the last floor of Crossbone Isle, which cannot be entered without beating the game. Iris is guarded by Dullahan the strongest enemy in the entire game, who must be defeated in order to acquire the Summon Tablet. Since there are likely no bosses remaining after Dullahan's defeat, Iris is meant more as a trophy or a "bragging rights" summon.

Trivia

 * While the Runes that appear when a Summon Tablet is activated are part of several real scripts, they don't form any real words but are instead random gibberish.