1. Is Saligram in the form of black stone only ?

Yes. Mostly saligram are black stone only. But there are other colour of saligram stone which are very rare and worshipping /keeping them in house bring more power and goodluck than normal black colour stone..

2. Is it available at gandaki river in nepal only or else where also ?

They are available only In gandaki river .

3. Does it have the symbol in saligram ?

Yes there are many symbol in saligram varies from chakra .... which represent the Different incarnation of Lord Vishnu ..

4. Is chakra a natural formation in slaigram ?

The chakras on the stones are formed by the river worms called Vajra-keeta. Its teeth/nails are said to be harder than granite stone. The chakra formation is found not only on the outer surface of the Saligrama. There are chakras inside Saligrama also.The Vajra keeta worms drill the stones, get inside and live there. During their stay inside the stones, they cause these chakras. Similarly outside Chakras also are caused by the worms only. Saligrama are used as living place by the worms not used as shells. " The puranic declaration is that the Lord Himself causes these chakras in the guise of worms. He reveals His presence in Saligrama through the Chakras.Because He is already present there by his own choice, We do not perform any pratishta-ritual for Saligrama just as we do for other idols."

5. Is it a fact that more number of chakras in a Saligrama represnet its significance ?

Yes. A Saligrama with more chakras is as powerful as the one chakras.

6. Any other mythological background to substantiate the prevalence of such stones deserve worship as the manifestation of Lord Vishnu ?

Of the 18 Puranas, a good number of them, under different contexts, proclaim the mahima of Saligrama as Vishnu pooja.

7. Is any size prescribed for the worship in the daily puja?

No . Any size or shape saligram can be kept in and worshipped..

8. Is it true that Saligram should not be kept in the house without daily Abisheka ?

The emphasis is only to stress the point that you should not be lazy and miss Saligrama Puja for too long a period on False excuses. It is not to ask you do away with Saligram on grounds of imaginary problems.

9. Is it correct that Broken saligram silas should not worhsip?

No. You are absolutely wrong. According to shastra " Even if a Salagram is damaged or broken, all sila are good to worship. "
" Khandita Sphutita Viina Vandi Dakdhya Tathi Va Cha Saligram Sialyatra Tatra Dosho Na Vidyate "