Hello, thank you for your help. I do not believe the small stones are diamonds as they are not set right to be diamonds. The color, shine and cut do not look right to be diamonds. I have no way to say 100% without testing the small stones. The stones that are in question are small and darker stones and I would believe them to be: Cassiterite, While used mostly in tin production, fine specimens of the black rocks are also used as ornaments. The black gemstones are usually opaque, but can be translucent when found in thin crystals. The black stones have a metallic luster that makes them desirable for jewelry.
The center stone is: Purple Topaz
While most popular in yellow and blue, topaz also occurs as purple stones. A variety of imperial topaz, this purple crystal is a rather rare color of the topaz mineral. Varying from light violet to deep purple, the violet crystal gets its color from the inclusion of chromium into the otherwise colorless mineral. Topaz in general is the gemstone of the zodiac sign Sagittarius.
The chain is sterling and is marked as such. The metal on the pendant is a base metal. A base metal is: Any non-precious metal other than the "Noble Metals": which are Gold, Silver, and the Platinum group metals. Copper, Nickel, Bronze, Brass, Stainless Steel, Aluminum, Lead and combinations of these are base metals often used in the manufacturing of fashion jewelry.
From the design and size of this pendant I would say is from the 1920-1930s. It is a very beautiful piece. The value would be 85.00-125.00 for this on the secodnary market today. Places to sell would be rubylane.com , etsy.com and also your local antique stores sometimes will purchase items from the public. Thank you and if I can be of further service please let me know, I am always happy to help. Thank you again.