Prince Michael and Princess Alexandra own all of the jewels in their possession, not the Duke of Kent. I wrote a letter to Kensington Palace years ago regarding the issue of Princess Michael's jewels and whether they belonged to the Queen or to her (I had just started being interested in the Royals and didn't know how these things worked). I received a very nice letter back which stated that the jewels were the personal property of Prince Michael inherited by him from his mother.
Princess Michael of Kent | Argyll Emeraldbrooch with Diamonds
Duchess of Kent | Diamond Daisies
Princess Michael of Kent |Pearls and Pearldrops of the Grandduchess Vladimir
Here are details about some of Princess Michael's jewels. Some belonged to Queen Victoria's daughter, Princess Louise.