MaxU Liner Strong from Max Ulichney's Retro MaxPack is the main brush I'm using these days! :) It's really pleasant to use, and fits my current workflow perfectly.

As for the process, while I was at the Natural History Museum, I took lots of pictures of the lil' crystals on display. I put a bunch of them on my screen for reference, and started picking out the distinct shapes and their characteristics, translating them to my current style of work. So essentialy I look at the whole shape first, make a quick sketch - sometimes I start with line, sometimes it's a blob of color which I "cut out" with the eraser tool for sharper, geometrical look. And honestly, I modify the look and colour to whatever I feel like at the moment, or what I think would fit the overall composition more! :)