Eigentlich hatte Kim schon die Antwort in seinem Diskussionsbeitrag gegeben, aber da dies nicht als förmliche Antwort gepostet wurde, will ich dies tun:

Maxisil N Natural Stone Silicone sealant
Non staining neutral cure silicone designed for expansion joints in natural stone e.g Marble & Granite
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(Use only Maxisil N smoothex especially for Natural Stone silicone)
http://www.robertsdesigns.com.au/silicones/maxisil-n.html

Natural Stone Silicone
Single-component, UV-resistant, water-proof, fungicidal, permanently elastic (will stretch up to 20 %) sealing compound on silicone base.
http://www.murexin.com/en/main3/sub2/05520/index.shtml

Dow Corning 795 Natural Stone Silicone Building Sealant 795NS
http://www.thecollegetoolbox.com/dowco795nast.html

For interior and exterior use. Not suitable for aquariums. Not suitable for jointing natural stone or in underwater areas. Here it is recommended that the natural stone silicone ECOSIL-2000-ST or the underwater silicone, ECOSIL-2000-UW are used.
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pure, unmodified acetate curing silicone sealant
http://www.schomburg.de/cmspdf/--53007-es.pdf

An unusually broad range of sealants and adhesives always guarantees to provide an optimal solution for you, also and particularly for special applications and difficult tasks such as Ottoseal S70 the number one silicone for marble and natural stone.
http://www.byds.co.uk/ottochemie.htm

The transition between floor tiles and skirting as well as the perpendicular joints in the corners of the room must be grouted with natural stone silicone based grouting material.
http://www.quickstone.co.uk/Laying System.htm

Arbosil XL 1099 is a One-part Low Modulus Neutral Cure Natural Stone Silicone Sealant, which cures on exposure to moisture vapour to form an elastic silicone rubber.
http://www.resapol.com/new/datasheets/data981.pdf