Semecarpus anacardium is native to India
It is known as bhallatak in India and was called "marking nuts" by Europeans, because it was used by washermen to mark the cloths before washing, as it imparted water insoluble mark to the cloth. It's also known as "Ker" in Kannada.
It is a deciduous tree, found in the outer Himalayas. The nut is about 2.5 cm long, ovoid and smooth lustrous black. In Ayurveda, the fruit is considered a rasayana for longevity and rejuvenation,and is processed before use, as it is toxic in nature.
Recent studies have shown the fruit to be a good anti-inflammatory agent and effective in various types of cancers.