What made India the jewel in the crown?

Very Simply, India was rich with resources, manpower and space compared to England, a nation which made its way into the sub-continent during the course of the 1800s and beyond, and she exploited India's resources and people, and left behind herself a very great deal of economic ruin and human suffering. England was able to finance its conflict with Napoleon with taxes raised in India.
India was rich with much that England could not produce as cheaply, or at all, because England could treat Indians worse than English people, and because the could pay them little compared to what they had to pay English people. Silk, indigo and fine cotton were among the main commodities that England got from India, part of which, that the English did not sell to other European nations nor consume itself, the English sold back, as manufactured goods (textiles), to Indians.
Realistically, India was the Jewel in the British Crown because India gave so much for relatively little English effort, and the English people profitted very greatly.