There was a reference to The Jewel in the Heart of the Lotus in an article on The Vipallasas by Ajahn Brahmavamso. It’s the transcript of his lecture on January 10th, 2001, titled:

The Vipallasas The distortion of view, perception and thoughts perpetuating delusion. It can be found at:

bswa.org

The reference is on page 9. That whole article is worth looking at.

The jewel in the heart of the lotus has various meanings if you do a search but I always stick close to Theravada or Dzogchen sources.