Indeed you are right. The Duke ordered the ring to the crown jewellers Leitão e Irmao and it is full of symbolism.
The enamel on the top is blue and white. These were Portugal's colours since before it's foundation.
This was the last flag of the County of Portugal:
And this was the first flag of Portugal as an independent Kingdom:
The last flag of the Kingdom of Portugal also had a very important presence of white and blue:
On one side (the visible in the picture I posted) there are 13 diamonds. This number remembers the day of the wedding (13th May 1995) and at the same time remembers the Our Lady of Fatima of which the Royal Family is devoted (the day of Our Lady of Fatima is also 13th May).
On the other side of the ring there is the Royal Monogram.