"The greenish light seemed to be growing larger at last, and within minutes, the boat had come to a halt, bumping gently into something Harry could not see at first, but when he raised his illuminated wand he saw that they had reached a small island of smooth rock in the centre of the lake...The island was no larger than Dumbledore's office: an expanse of flat dark stone on which stood nothing but the source of the greenish light, which looked much brighter up close. Harry squinted at it; at first he thought it was a lamp of some kind, but then he saw that the light was coming from a stone basion rather like the Pensieve, which was set on top of a pedestal"
The island itself is a small expanse of flat dark stone, no larger than the Headmaster's office at Hogwarts. In the middle of the island is a basin full of an emerald green potion, which must be drunk in order to get the Horcrux at the bottom. The potion would cause nightmarish delusions and extreme thirst, which would force the intruder to drink from the lake, as any other water produced would vanish, thus activating the Inferi defence.
Behind the scenes
In the film adaptation of Harry Potter and the Half-Blood Prince, the island seems to be made of crystals of rock salt (even though crystaline salt formations are evaporites and would dissolve in those circumstances), inspired from a salt crystal cave in Frankfurt, Germany. This directly contradicts the novel, which describes a smooth dark stone formation.