Simplify the following text for a Grade 11 native English-speaking person.
Original: Among the rocks then obtained and submitted to Sir John Murray for examination were many of nearly pure phosphate of lime , a discovery which led to annexation of the island by the British Crown on 6 June 1888 .
Simplified: There were several visits and explorations of the island , but it was only when valuable phosphate of lime was discovered that the island was annexed to ( claimed by ) the British Crown on 6 June 1888 .