What does "tea ring" mean in this sentence and why "ring"?

Question

I could not understand what is "tea ring" in this sentence, all I searched resulted in a "Swedish tea ring", as bread or cake, and I am pretty sure it is not.

My guess is : it refers to some kind of tea marks/stains left on the mugs? ? But WHY use the word 'ring'? I could not find any explanation in the dictionary referring 'ring' as a stain or mark or something. Could any one help me to get a glimpse of the sentence?

Here is the sentence in the context:

I had finally begun to understand the consolations of cleaning. I mopped, cleaned windows and changed duvets. I was a constant whirl of activity. No dust mote escaped my eye, no tea ring my forensic attentions. I was dislodging the limescale on the bathroom taps using kitchen roll soaked in vinegar (my mother’s tip) when I heard Will’s chair behind me.

Me Before You by Jojo Moyes

Answer 1

I'm pretty sure that, with the context of cleaning, the author is talking about the ring of liquid left on a table when a cup is moved. While these are often from condensation on glasses of cold liquids like water, they can also come from tea cups and coffee mugs (and even wine glasses) that have sloshed slightly onto their saucers and gotten on the bottom of the cup. When set on a table, they leave a "ring" like this:

With water, these aren't necessarily visible since there's no color, though it may damage the table's surface. With tea, coffee, and wine, it's about the slight discoloration caused due to the liquid's color - brown for coffee or tea and red for (red) wine.