Rock Crystal ore can be found deep underground at Y-level 4 and below (give or take a few levels up or down) using a Resonating Wand. It requires a Diamond pickaxe (this could be different depending on other mods installed; needs to be able to mine obsidian) to harvest. A warning: Rock Crystal Ore can only be mined by a player standing within 10 blocks of it, anything else (like a quarry) and the ore will drop nothing.
You are going to need more Rock Crystals. These are not always easy to come by so use these sparingly. If you wish to make a sword or a pickaxe from Rock Crystal take the following into consideration:
You can use normal Rock Crystals but you can enhance these to make them more powerful. Throwing these into Liquid Starlight will increase the Crystals overall size (up to 400). This is a slow process and could take some time so while that is happening you can move onto making a Grindstone.