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With this descriptions it seems to be a sebaceous cyst
,which is nothing but a nodular lesions with in the skin and at times it gets calcified to form a stone like mass.
Nape of neck is the most common place to have it.
This would require a small surgery to get it removed .There is nothing be scared of as it is a very benign condition.
What it contains is nothing but sebum.
The other possibility is of LIPOMA
,which is nothing but a fat lobule with in a bag.
This too can be excised and removed surgically.
Kindly show to a dermatologist