The planet was chosen due to having “crystalline deposits.”
The First Order selected the icy world for its unique energy transmitting
The Force Awakens Visual Dictionary
This would be pretty persuasive on its own, given the history of Dark Side superweapons and kyber crystals. But, to remove any doubt, this tweet from the official Star Wars Twitter account explicitly confirms that Starkiller Base was built on a planet with plentiful kyber crystal deposits:
Presumably these are the same “energy-transmitting crystalline deposits” for which the First Order selected the planet, which all but guarantees that they used them in their superweapon.
The text of the tweet is even more explicit:
The Empire’s hidden resource became the First Order’s ultimate weapon.