Fergie's ring, a Burmese ruby, reportedly cost the prince $33,000 back in 1986.
After a whirlwind romance, the couple announced their engagement in March, 1986.
The engagement ring, which features a similar design to Princess Diana's Garrard ring (now worn by Kate Middleton), has one large ruby surrounded by 10 diamonds, forming a flower pattern on a yellow gold band.
When asked about the ring in their first interview following the engagement announcement, Fergie described it as 'stunning' and 'red.'
The decision by Andrew to include a ruby had a deeper meaning for the couple.
'We came to the mutual conclusion that red was the best colour for Sarah,' he said at the time. 'That's how we came to the choice of the ruby.'
Fergie continued to wear her engagement ring years after her divorce from Andrew, and recently the two have been urged by fans to 'reunite.'
It's unclear if Fergie or Andrew now have the ring, and it has not been passed down to either of their daughters - Princess Beatrice and Princess Eugenie.
However, Princess Eugenie's sapphire engagement ring is very similar in design and colour.