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Silicon also has a black streak and a hardness of around 6.5-7.  Part of the issue here may be your streak plate.  Make sure you use an unglazed porcelain tile.  Scratch resistant porcelain tile should have a hardness of at least 7.  Non scratch resistant porcelain tile can be as soft as 4.  You may need to test your tile to verify it is 7 or higher.

Then, you said the rock would shatter a porcelain tile.  I may be drawing a wrong conclusion from that statement.  Many people mistake hardness and toughness.  You only need to “draw” on the tile (like with chalk) just hard enough to leave a streak.  Pounding and grinding are counter-productive here.

Also, if this rock is harder than your bit, and drilling did not “bother” the rock, then the  “grounds” from drilling may have been dust off your bit.  If so, the lack of a smear does not mean the rock lacks a streak color.  I am not familiar with drill bit hardness ratings.  How does your 5.5-6 mach rating equate to the Mohs scale?

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