The images you perceive are the shadows on your retina of whatever may be floating in the vitreous of that eye. Just google "circular floater" and their image bank.
1. The dark central portion and gray surroundings around it can come in general from the different types of shadows that can be cast on the retina by the light from the outside on that floating body, called umbra (the dark central portion) and penumbra (the lighter shadow around it). This you may know from solar eclipse phenomenon - where say the moon is between the earth and the sun, and there is a region of dark shadow of the moon on the earth (umbra where no light reaches) and lighter shadow (where some of the light rays from the sun reach but not all). The above is true of whatever the shape may be of the floating body, but the relative values of the dark and gray areas depend on the body's distance from the retina, as you can quickly see by the shadow of your finger casted on the wall, and moving your finger towards the wall.
2. Typical floaters are, indeed, like hair, cobwebs, gnats, etc., floating around in the vitreous (stuff between the lens in the front and the retina at the back). However, the specks can sometimes be something else, like, when I had retinal tear and retinal detachment, or after my retinal repair surgery, my dr said that if they are solid black dots or of some odd shaped black particles then they can be blood cells. This is something your dr needs to diagnose. I have seen exactly what you are saying - small black specks with gray circles. So you need to see an opthalmologist just to check if it is not due to something more serious.
3. Lastly, talking about "circular" floaters, there is something called a "Weiss ring," which is much larger. A Weiss ring floater forms when the vitreous tissue that surrounds the optic nerve in the back of the eye separates from it and floats around freely in the vitreous humor - a process known as vitreous detachment.
Regardless, you need to see your opthalmologist first. It could just be innocuous floaters...