"In the ancient world a white stone was used as an admission ticket to public festivals..."
(Jewish New Testament Commentary, David Stern)
"The 'white stone' noticed in Revelation 2:17 has been variously regarded as referring to the pebble of acquittal in Greek courts; to the lot cast in elections in Greece: to both these conbined, the white conveying the notion of acquittal, the stone that of election; to the stones in the high-priest's breastplate; to the tickets presented to the victors at the public games, securing them maintenance at the public expense; or lastly to the custom of writing on stones."
(Dr. William Smith's Dictionary of the Bible, 1872)
The white stone represents 'a pebble.'
(Hastings' Dictionary of the Bible)
From Webster's Dictionary:
(This beautifully illustrates the cleansing action of Jesus, who we run to when we thirst.)
"During the time in which John wrote, a white stone was used in social life and judicial customs. Days of great festivals were noted by a white stone, days of great calamity were noted by a black stone. If a host had a special guest whom he greatly appreciated, the guest was given a white stone with a name or message written on it, directed only to the guest--not to be shared by any other."
(The Revelation Verse by Verse Study, Dr. Oliver B. Greene)
"...the allusion is to a custom among the Jews, who used to examine the priests and Levites before they went to their service, or to the sanhedrim, to judge and pass sentence, whether their ways and works were right; and if they were as they should be, they gave them 'the stone of the sanctuary': if not, they might not enter on business..."
(The New John Gill's Exposition of the Entire Bible)
"...a white stone, and in the stone a new name written..."
"Even unto them will I give in mine house and within my walls a place and a name better than of sons and daughters: I will give them an everlasting name, that shall not be cut off"--Isaiah 56:5
"and the Gentiles shall see thy righteousness, and all kings thy glory: and thou shalt be called by a new name, which the mouth of the Lord shall name"--Isaiah 62:2
"She [Zion] is called by a new name; and who could determine this new name: He who makes the church righteous and glorious, He, and He alone, is able to utter a name answering to her new nature, just as it was He who called Abram Abraham, and Jacob Israel."
(Keil & Delitzsch Commentary on the Old Testament, Isaiah, volume 7)