لمادا كتبت التاء مفتوحة في كلمة الياقوت

The letter “taa” is written as an open “taa” in the word “yaaqoot” because it comes after a sukoon. The open “taa” is used in nouns when it is preceded by a consonant, in this case the letter “waw”. The rule for writing the open “taa” is applied because of the preceding consonant “waw” in the word “yaaqoot”.