四分律行事鈔資持記 3.631

3.631

二中。謂以木鑽鑽木，則有火出。火珠，即水精珠，日光照之，用艾引火；恐同出術，故開屏用。

In the second: It means that by drilling wood with a wooden drill, fire comes forth. A fire pearl is a crystal sphere; shining sunlight through it, one uses moxa to kindle fire; fearing that it may seem identical to miraculous arts, it is permitted to use it in private.