Without a picture it is hard to say. But if you have a forest green island and creamy cabinets your best bet is a granite or quartzite with creamy browns and maybe golds, or would you want a really simple creamy quartz? What color is your hardware and what kind of backsplash?

Soapstone

Here's the picture! So dark hardware and unsure of backsplash. We will only have a small amount by the window. There will be a farm sink underneath the window and our cooktop will be in the island. Thank you for the suggestions!

Very pretty! I think you might be starting a trend, because I’ve not seen a lot of forest green islands but I love it!

You could actually go white-ish gray for countertops. If you’d like to stay simple, look at some quartz or silestone or if you want a little more movement, there are a lot of granites that would complement. Not exactly your color green but goes well...

We have Fiore Bianco granite & it would look great in your space. We also looked at Colonial Breeze & Colonial Cream that both would be good choices for you.

The sink is white? What's your floor?

Creamy white base with grey, taupe, or brownish veins would go. Lots of quartz options to fit the bill, real marble would work, taj mahal quartzite

I am a fabricator. Given "very farmhouse/rustic" I'd also suggest soapstone. It's a great countertop material and fits your "rustic" décor perfectly. Soapstone was once the countertop of choice for the affluent because of it's beauty and substantial durability advantage over marble and limestone. Back in the day those stones were the only ones that could be worked with the tools available at the time.

I would do butcher block on the island

I would NOT do a different counter on the island and the perimeter. One countertop since you already have a different colored island. If you do a different island counter from the perimeter, that will be 4 different materials in the kitchen. Where will your eye focus?

I also agree that your best bet is soapstone. It would be perfect for the look you're after.

Soapstone for sure would really ground it!