Regardless of where I procure my chia seeds, I always find, mixed with them, some quantity of smaller and darker seeds which don't seem to be chia.
These seeds are black (noticeably darker than the dark-gray chia seeds) and lentil-shaped, about 1mm in diameter. They don't gel in water like the chia seeds do.
What are these seeds? How safe are they? Should I make an effort to get them out?
Here's a sample, the 3 little seeds on the top right are the kind in question, the rest are chia:
Exactly which species they are I don't know, but it will probably depend on where the chia seeds are sourced from in the world. For a possible ID you would need to provide more close-up photos of the seeds, showing them opened out and showing the bit that attaches to the seed pod. They could well be one of the many common Brassica species that are weeds and/or crops of many places, given their small round shape, but this is by no means certain.
What they are is contaminant species that have either been planted in the same fields as the chia previously (i.e. crop rotation), or have grown adventitiously as a weed species - either already present in the fields or transported and incidentally planted with the chia.
The reason they are in your chia is that the sorting and cleaning process can't sort them out from the chia seeds in any meaningful, cost-effective manner.
As to safety - Without an ID it is almost impossible to say. However, if the producer hasn't gone to great lengths to remove them, they probably aren't likely to cause you harm, at least at the level at which you find them in the chia.
The black seed looks like a kalonji or nigella seed to me.
