If you read that article though my first suggestion is to use dry cleaning solvent to try to remove the stain, in case it is oil based in origin. That is because water can sometimes set oil based stains.
No matter though, even after washing your dress it really wouldn't hurt to try dry cleaning fluid or solvent now on the stain.
You can also follow the other steps in the article about removing mystery stains I mentioned above, such as presoaking and trying various acidic and alkaline stain removal solutions.
Finally, I have one more suggestion since the item has already been washed.
You didn't mention whether you soaked the dress in chlorine bleach or color safe (or oxygen) bleach, but you may want to try soaking the item in the bleach solution again, but for a longer period of time.
I wouldn't soak cloth in a chlorine bleach and water solution for more than an hour or so, because chlorine bleach can start to harm the fibers.
However, I have often soaked badly stained clothing in oxygen bleach, such as Oxiclean for several hours, or even overnight. That extra length of time can sometimes help with really stubborn stains that don't seem to be coming out any other way.
I hope these suggestions help. I would love it if you wrote back in with how they worked in the comments, and tell me what worked (or didn't).
Further, if any other readers have tips or ideas for Natalie's unknown stain removal problem, please tell us in the comments.
I hope you enjoy this gift, and stop by again soon!
CAUTION: This website is provided for informational purposes only. It is provided as is, without warranties or guarantees. Some stains and messes just won't come out, and are permanent. Further, some cleaning methods can harm your item, so if what you want to clean or launder is sentimental or expensive call a professional. See disclaimer of liability for more information.