A HS2/AIS head accessory.
Download the .zipmod file for the latest version on the Release page, and drag and drop it into your /mods/ folder or use KKManager to install it.
In Chara Maker Mode, go to the Accessory panel, and then the Head tab, search for "blake", "sailor moon" or "tiara" to find this accessory.
Add it to your chara, and it suggests changing its Base Position to "Forehead" so that it will be easy to place it in an accurate position.
It was rigged so that you can adjust its bones in Studio. Enable the adv.mode of HS2PE/AIPE, go to the Bone tab, search for "diamond", and you'll see the bones namely diamond.total, diamond.bottom, and diamond.top. (In addition, if you only want to rescale, rather than to rotate, the bones, you can use AIABMX to adjust them in Chara Maker Mode instead of in Studio.)
Select diamond.total, and scale it up. The whole jewelry on the front of the ring is enlarged. This bone affects diamond.bottom and diamond.top which are its children.
Diamond.bottom controls (and deforms) the golden base under the gemstone, but doesn't affect the gemstone.
Diamond.top controls (and deforms) the gemstone.
Also, you can search for "Bone", and there are Bone.L.1, Bone.L.2, Bone.L.3, Bone.L.4, Bone.R.1, Bone.R.2, Bone.R.3, and Bone.R.4 controlling (and deforming) the golden ring. You can rotate, move, or rescale them to make a different pose, especially when the tiara clips into the chara's head.
The bones referred above are distributed like the screenshots:
