If I needed to choose my favorite pen ever it will be Posca Pens.
They have a smooth, paint-like effect and work great not just on rocks but on most surfaces!
For small sea glasses, the ultra-fine Poscas are the best. Also, they are the best one to create detailed designs on rocks. Its a must-have!
For more ideas:
Instagram Word Rocks Project
Instagram Word Rocks Project
Pinterest Word Rocks Project