You can’t rely on any “diamond testers” unless it’s a Presidium. They test thermal transmission, anything off eBay or from Amazon with “coloured lights” is unreliable.
Still, the tests a gemologist would use would include specific gravity, LW and SW, spectroscopy and hardness.
the chances of finding a raw diamond crystal that size are less than being hit my a meteorite. Seriously.
if by some miracle it was actually diamond, despite its size it’s opaque. Diamond crystals need to be transparent AND free of inclusions to be facetted into commercial gemstones.
i Think it’s Quartz.