This is a 2.7 oz bottle of iron gall ink. Iron gall ink (also known as oak gall ink) is a purple-black or brown-black ink made from iron salts and tannic acids from oak galls. It was the standard writing and drawing ink in Europe, from about the 5th century to the 19th century, and remained in use well into the 20th century. Its consistency and color is much different from that of india ink or other modern, commonly used inks. The consistency can be very dark and thicker to light and washy (water can be added to create more of an ink wash). The color can vary from black, black-brown, dark purple, or dark navy. Another unique feature to this ink is the possible change to color and/or corrosion over time due to the organic matter and iron within it.
This kind of ink has an incredibly rich historical background and can make for very beautiful and unexpected ink wash drawings, or whatever else you use it for!