As stated a skein is ball of yarn a variable quanity
(think of a bag--)
yarn is sold by 2 measuremtents
Weight (50 gms) and Length (100 meters/109 yds)
(think of fruit.. 1 Kilo cherries is by count, way more cherries than 1 kilo of apples!)
the same is true for yarn.
50gms of sock weight (fine, thin yarn) is about 200 meters. (depending on yarn it could be as little as 150 meters or as many as 240!) (cherries)
50gms of worsted weight is about 100 meters (or less!)
50 gms of cotton would be even fewer yards/meters (cotton is a heavier fiber) (think of apples)
In many countries, there are requirements for sale of yarn
(weight, yardage, fiber content, country of origin)
but this is not true everywhere.
and frequently, even in countries that have requirements, yarn can be sold "as is" (at a very cheap price) with out a label--(and often is sold this way at sales/clearance bins. )
(let the buyer beware--you don't know what you are buying.. so you can't be sure of the value!)
the yarn is cheap--and it may be a bargain..
but its hard to know.
how many yards/meters in the 50gms?
what is fiber (there are several tests.. including a basic one like smell.. (wet smell is best) wet wool often smells (faintly!) like sheep. (other animal fibers so the same thing.. they smell like alpaca's, or goat (mohair--but very, very faintly!) or like, well, horrid (silk!)
there is a burn test too, (cut off 10cm of yarn, place in ashtray or other fireproof dish, and lite it on fire..
(wool and other animal fibers smell like burnt hair, and many will not remain on fire once the flame is removed. they have a fine ash, that will stay in place (till you touch it, and then, poof, its gone!)
synthetics will burn rapidly, remain on fire, smell like petrolium, and leave a small plastic beads for ash.
a photo would help, with a scale (a AA battery near by, so we could gauge how think or fine the fiber is (compared to a known size! )
And you can investigate for your self!