CREATE TABLE table_12107 ("Date" CLOB, "Tournament" CLOB, "Surface" CLOB, "Opponent in the final" CLOB, "Score" CLOB)

What surface was the tournament played on against Steven Diez?
SELECT "Surface" FROM table_12107 WHERE "Opponent in the final" = 'steven diez'
SELECT "Surface" FROM "table_12107" WHERE "Opponent in the final" = 'steven diez'