Iron gall ink formulations have been around for over 1500 years. They are produced by combing iron salts with tannic acids and create an ink that is completely waterproof. A really cool feature of these inks is they darken after the ink dries and oxidizes.
The primary disadvantage of iron gall inks is that they are extremely acidic and corrosive and can damage both pens and paper over time. Modern iron gall formulations are reportedly much gentler, however. That said, they are considered by many to be "high-maintenance" inks.
After reading many positive reviews, I decided to try Rohrer & Klingner's Scabiosa, an iron gall ink with a purplish hue. The only iron gall ink that I had used previously was Diamine Registrar's Ink and it was a bit dry for my tastes. R&K is a German ink maker that has been around since 1892 and produces 18 different fountain pen inks, including two iron gall formulas.
According to their web site: "Our inks feature high-class, brilliant colourants, specially treated water and minimal amounts of additives. This well-balanced composition causes the optimal cappiliarity of the inks and the accordingly good writing conduct. It is suitable for pens, quills and other calligraphy utensils."
I found Scabiosa to be a pleasant ink to use. It seems to be far wetter than the DRI that I used previously. With some very absorbent paper, I even experienced some bleed through and feathering. On cheap copy paper, the ink seems to be well behaved.
It goes down as a light lavender color but dries to a concord grape or raisin color (depending on the paper). I have been using it exclusively in my Indian ink dropper pens because they use simple ebonite feeds and are relatively inexpensive. In those pens, I have not experienced any serious clogging, although the flow may be a bit sluggish if the pen is not used or left uncapped for a while. In my soak tests, I found the ink to be completely waterproof once it dries.
R&K Scabiosa is a great ink and definitely worth a try!
P. S.: Be careful with this ink and don't leave it in pens with steel nibs.