Yeah you can exchange gems for stat gems, orbs and tomes of insights too. This is Akyuu's inventory for those who are interested:
Chest Key - 2 gems (almost definitely implying that locked chests will show up again, likely in endless corridor even)
Tome of Reincarnation - 5 gems
Orihalcon - 16 gems
Adamantite - 16 gems (those two are materials)
[A set of six single stat+400% subequips in the order of HP, ATK, DEF, MAG, MND, SPD] - 4 gems
[A set of four single stat+144% plus four elemental resist+128/four ailment resist+50] - 6 gems
Paladin Shield (DEF+450%, MND+225%, Evade+12, FIR CLD SPI DRK PHY+70, DBF+50) - 16 gems
Fairy "Navi" (MP+2 HP/DEF/SPD+166% MND+422% Evade+24 SPI+100 SIL+72) - 16 gems
Long Sword "Crimson Lotus Princess" (HP/ATK+412%, MAG/SPD+199%, FIR+144, DRK+72 PAR/TRR/DTH+40) - 20 gems
Flower Blade Kikuryusei (HP/ATK/DEF/MAG+368%, SPD+256%, PHY+50, DTH+50) - 20 gems
Gran Grimoire (MP+4, MAG+600%, MND+360%, All Elements+48) - 30 gems
Necronomicon (TP+6, HP+360%, MAG/MND+480%, All Ailments+24) - 30 gems
[A set of three normal reward increasers] - 4 gems
[A set of three alternative reward increasers] - 12 gems - These have the same magnitude and cap as the normal set, but they stack with the normal set and likely have their own cap
[A set of stat gems - HP, MP, TP, ATK, DEF, MAG, MND, SPD] - 2 gems
[A set of stat orbs - HP, MP, TP, ATK, DEF, MAG, MND, SPD] - 4 gems - these can only be used on characters who already used 10 gems of that stat, and have the same 0.2 bonus per orb
[A set of Tomes of Insight - HP, MP, TP, ATK, DEF, MAG, MND, SPD, Evade, Accuracy, Affinity, Resists] - 6 gems
[A set of Tomes of Vigorous Training - for learning second boosts] - 12 gems
[A set of Tomes of Endless Drilling - for learning mega boosts] - 24 gems