So, let me just state, that I love mineral type pokemon, especially Rock-Types, so bear with me here.^^ Also, might I suggest, that some crest are optainable or faced before the dessert instead of only in the late-game.
Anyway! I love this game, so I hope I inspire you with these. I haven't read ALL posts, so sorry if something was already mentioned.
Carbink - Doubles the Damage inflicted by Stealthrock if layed by a Carbink. Adds 35% of Sp. Defense to Sp. Attack.
OR Carbink - Automatically activates the effect "Trick Room" for 5 turns upon switch-in. Adds 35% of Sp. Defense to Sp. Attack.
OR Carbink - Gains Ability "Shields Down".
Kabutops - Additionally gives it the Ability "Adaptability".
Lunatone and Solrock - If together on the field: Gain Plus and Minus respectively (or sth. similar with Attack for Solrock^^). When using Psychic type moves, adds 50% of the Attack or Sp. Attack stat (depending on move) of the PARTNER to the Power. (dunno if that is even possible.)
Coalossal - Becomes immune to Water-Type attacks (or at least only normal effective). Also, when hit by an opposing Attack, automatically inflicts "Tar Shot".
Aurorus - Ice-Type moves (inkluding Normal-Type moves that have become Ice-Type) have +1 Priority. Speed increased by 1.3.
Probopass - Sp. Attack increased by 50% of Sp. Def. Automatically uses (or simulates using) the move Gravity after being hit by a Ground-Type move.
Aggron - Gives it the Ability "Rock Head" if Ability is "Sturdy", "Heavy Metal" if "Rock Head" and "Sturdy" if "Heavy Metal". OR Gives it "Reckless".
Armaldo - What "Nooooooo" said: Water Stab, pls.
Gigalith - Gives it the Ability "Solid Rock". Also, the move "Smack Down" has +2 Priority.
Cradily - Defense and Sp. Defense are increased by 50 % of its combined Attack or Sp. Attack (whichever is higher) and Speed.
Rhyperior - Gives it the Ability "Solid Rock" or increases its value. (With 2x Solid Rock -> 2x super effective = 1x effective. 4x super effective = 2x super effective)
Donphan - Becomes part Steel-Type. Speed equals HP stat OR is increased by 40-50% of HP stat.
Golurk - Moves that take 2 turns only take 1 and have 30% increased Damage (Like Phantom Force and Dig; MAYBE Focus Punch?). Also, Sp. Attack is equal the Attack Stat.
Hippowdon - Gains Abilities "Strong Jaw" and "Early Bird". Also, Speed equals Sp. Defense.
Nidoking and Nidoqueen - Switches Attack and Sp. Attack. Increases Non-Stab moves Base-Power by 1.3 .
Cursola - Becomes part Rock-Type and Switches Speed with Sp. Def, but decreases Sp. Atk by 15.
Gogoat - Becomes part Rock-Type, gains Ability "Rock Head" and Normal-Type moves are turned into Rock-Type moves.
Sorry for my Rock-Type fixation.^^ I just love this type, even if it's screwed in terms of weaknesses.
Thank you for all your hard work and good luck on development.
