Whenever I grind coriander seeds the outside seems to separate like a husk and doesn't grind well.
I use a pestel and mortar.
Is there a trick to getting finely ground coriander?
I recommend a dedicated, rotary style coffee grinder for home spice grinding. Whole spices last longer, usually taste better, and are often less expensive than pre-ground. In many food cultures, whole spices are often toasted whole, then ground. It makes a big difference for not a lot of effort.
I picked up a rotary style coffee grinder for $3 at a thrift store.
I use it to grind all kinds of spices. It never needs to be washed. If it starts to taste like last weeks spices, I just grind up some rice and shake it out. It works great, I've ground spices in the same grinder for 5 years.