This is a natural smoky-yellow star quartz cabochon from <st1:country-region w:st="on"><st1:place w:st="on">Brazil</st1:place></st1:country-region>.  The star is caused by microscopic crystals
within the stone.  With proper cutting
the star is revealed when the stone is viewed in sunlight or under a light bulb
(not fluorescent).  The little "star" at left is a reflection.  Fine quality stones
are quite hard to find.  It hasn’t been
treated in any way other than cutting. 
It has a flat bottom, excellent polish and measures 15 mm. x 9.5 mm.
deep.  (All uses, N)