This is one of the 2 MAC polishes I own. I got this when i was in Canada over the summer, and I absolutely adore it. My mother even loved this polish, and she normally tries to talk me out of buying it!
Formidable! is a dark purple based with duochrome glass flecks in it. The flecks range from fuschia to a turquoise blue, and the transition here is very obvious on the nail. I believe this was two coats, maybe three. It applies very smooth for such a complex polish, and I always get compliments on this when I wear it. The base almost has a tiny bit of earthy brown to it, making it all that more complex. I've never tried this over black because the base itself is dark, but I may try it to preserve the rest of this expensive nail varnish.
I believe this was around $22 CAD, not sure what it is in the states/ if it is available here. This was the first time I had ever seen or heard of the polish, so it may have been a Canada release only. Let me know if any of you guys have this or if you know more about it!