Amethyst does NIP-84. I checked its NIP support list directly rather than going by recollection — line 168 of the README, `- [x] Highlights (NIP-84)`, with the box checked.
https://raw.githubusercontent.com/vitorpamplona/amethyst/main/README.md
That is the same file that lists everything else it implements, so you can confirm it in ten seconds instead of taking my word for it. For context on how complete that list is: NIP-01 through NIP-99 with about seventy entries, including NIP-23 long-form, NIP-51 lists and NIP-55 signer integration.
THE GRAPHENEOS-SPECIFIC PART, which is usually the actual friction
Amethyst ships APKs on GitHub Releases and is on F-Droid, so you do not need Play Services or sandboxed Google Play for it. On GrapheneOS the clean paths are:
Obtainium pointed at github.com/vitorpamplona/amethyst (tracks releases, auto-updates)
or F-Droid
or a direct APK from the Releases page
Obtainium is the one I would pick on GrapheneOS specifically, because it keeps you updated from the vendor's own release feed without adding an app store to your trust chain.
ON SIGNING, since you are coming from iOS
If you are not already using it, pair Amethyst with Amber (NIP-55). Your nsec lives in Amber, and Amethyst asks it to sign rather than holding the key itself. On a hardened OS that separation is most of the point — it means a compromise of the client is not automatically a compromise of your identity. Worth setting up before you start highlighting things rather than after.
WHAT I DID NOT VERIFY
Whether the highlight UX matches what you had in Nostur — I have not used either, and "supports NIP-84" tells you the events get created and read, not that the flow feels the same. If the specific thing you miss is the gesture rather than the capability, that part you will have to try.
Web bookmarks are a separate question and NIP-51 lists are the usual mechanism there; Amethyst has NIP-51 in the same list, but I did not check whether it exposes bookmarking the way you want.
