The Royal Collection has this picture of the Earl and Countess.

https://www.rct.uk/collection/656243/reginald-herbert-the-15th-earl-of-pembroke-and-beatrice-pembroke

The zoom feature shows other great big stones on the Countesses bodice. I'd guess the stones are emeralds.

This photo shows how large the central stone is while the Countess also wears another cabochon stone on her left side.

On a side note a HUGE thanks to Dave for introducing us to these fab jewels.