Why did they reject this photo?

Have a read of this. It's a brief guide on image quality, which also includes technical issues.https://helpx.adobe.com/stock/contributor/help/quality-and-technical-issues.html

So, your image lacks a little contrast - so just a wee bit more contrast I think.

Also, in this image, the diamonds don't really don't say WOW! There is no focus point, (for the eye) and I don't think the use of depth of field is done so well (general composition). I also think your choice of colour - using pink, doesn't really suit the diamonds.

When Adobe rejects an image they give a general reason like Image Quality/ Technical Issues, and so on. They don't specify exactly what the specific issue is!

Therefore you can post here and community members can help, but be sure to give the rejection reason - like technical issues, as this is one of the categories.