समरागलेखमनिम्नोन्नते देशे निकषितम् परिमृदितं परिलीढं नखान्तराद्वा गैरिकेणावचूर्णितमुपधिं विद्यात् ॥ कZ_०२.१३.१८ ॥
samarāgalekham animnonnate deśe nikaṣitam parimṛditaṃ parilīḍhaṃ nakhāntarād vā gairikeṇāvacūrṇitam upadhiṃ vidyāt
If the streak appears uniformly coloured, yet—when tested on a spot that is neither raised nor hollow—it becomes smeared, dragged off, can be scraped into the nail-groove, or shows an ochre-like powdering, one should recognize it as adulteration (upadhi).
To control the test environment: uneven surfaces can distort the streak and create false positives/negatives in adjudicating fraud.
It operationalizes enforcement: officers can justify seizure/penalty using observable criteria rather than discretionary opinion.