tehuilotl.

Headword: 
tehuilotl.
Principal English Translation: 

glass, crystal (see Molina and Karttunen)

Orthographic Variants: 
teuilotl
IPAspelling: 
tewiːloːtɬ
Alonso de Molina: 

teuilotl. cristal, o vidrio.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 112r. col. 1. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

TEHUĪLŌ-TL pl: -MEH glass, crystal / cristal o vidrio (M) [(1)Tp.224].
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 219.

Attestations from sources in English: 

Yn iuhq’ in tonatiuh ytlanex yiticpa ualq’ça in teuilotl in amo quẽ mochiua in amo yc tlapani in amo ca tçayani yn teuilotl = It was like the way that the light of the sun comes forth from inside crystal. Nothing happens to the crystal, it does not thus break, it does not tear (early seventeenth century, Central Mexico)
Louise M. Burkhart, Before Guadalupe: The Virgin Mary in Early Colonial Nahuatl Literature, Institute for Mesoamerican Studies Monograph 13 (Albany: University at Albany, 2001), 59.

Tehuilotl (glass, crystal) can come in various colors, and therefore it will appear in compound words such as this one for amethyst. Book 11, f. 206v, of the Florentine Codex has a discussion of this. One type of amethyst, apparently with a red tone, is called tlapaltehuilotl. See: https://florentinecodex.getty.edu/search?term=tehuilotl&view=text&filters=

themes: 
Jewelry, Adornment, Accouterments