Please accept my sympathy on the loss of your mother. Following are four verses which may be appropriate for her headstone.
(1) Blessed are those who die in the Lord . . . for they will rest from their hard work; for their good deeds follow them! (Revelation 14:13 NLT)
(2) Precious in the sight of the Lord is the death of His godly ones. (Psalm 116:15 NASB)
(3) We remember before our God and Father your work produced by faith, your labor prompted by love, and your endurance inspired by hope in our Lord Jesus Christ (1 Thessalonians 1:3 NIV).
(4) Those who are righteous will be long remembered (Psalm 112:6 NLT).
Hope this helps.