A plain mercury glass ball, 10 inch, is offered at one site for $US230. Colored ones are worth more.
Those with mottled or varied colors, often known as 'witch balls,' can be worth a great deal. A 5-inch multicolor is offered by a dealer for $1200.
The Sandwich Glass Co. and other 19th c. glass makers produced them, some as 'whimseys' which often are one-of-a-kind, but the simple globular forms were made fairly abundantly for decorative purposes.

I remember a neighbor, years ago, who had one on a stand, an eye-catching decoration.

Made in England from mid-18th c. and in U. S. and Canada in the 19th c.