If you do the tomb properly [starting with the giant boulders at Petit Bois] there are plenty of rare grapes to harvest as you work your way down to the lowest level. They're not always easy to spot due to the low light.
Once you have used the dive pool to get access to Tuatha's Garden [underneath the little Island] you will find Death Flower, Flame Fruit, Life Fruit, Money Trees and more rare grapes ... harvesting the whole lot can take quite a while.
Then, making your way up to the surface, there are a couple more rare grapes before finally emerging on the surface of the island for more rare grapes, up to 4 workable 'excavate' sites and assorted rocks, gems and butterflies.
PS ... if you teleport/zeneport to the island, the stairs going down won't be revealed. They only appear if you work your way up from the bottom.