What is this stone/crystal?

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Hi! I have several of these cabochons. They are all Clear (glass like) except some have black "threads" through them as well as some white/frosty looking threads and spots. Some have more, some less "threads" My guess is that these are Tourmalinated Quartz. Does anyone have any suggestions? They feel like glass so I'm guessing they've been "tumbled". Could they be fake too? ANY suggestions/assistance would be greatly appreciated.
-Christine
PS: The pix look more cloudy. They are actually pretty clear.
q help me identify this stone crystal, concrete masonry
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