Sincerity, Diligence, Effort
Noun and する verb
たんせい
お婆ちゃんが丹誠込めて育てたトマトだよ。
Grandma worked earnestly and raised the tomatoes.
A miner works with such sincerity that her arms and legs start turning rust colored from working earnestly. All the clay and rocks she's putting her effort into digging are turning her the same color. That color is how you know someone works with serious diligence.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.